Question :
I want to access my Synology DS218 NAS because I got an eMail from Synology that there is a new system update available, but when trying to login I need the Synology Secure SignIn app on my iPhone. But I haven’t used the app for a while, and now it turns out it has been offloaded by iOS. I’ve re-installed the app, but now my entire account has been erased and syncronisation with my NAS is no longer possible.
I tried to setup my account in the Secure SignIn again, but when trying to do so, I get caught in a loop… I need an access to the app to login to the NAS’ virtual desktop interface… and I need access to my NAS’ virtual desktop interface to setup an account in the Secure SignIn app…

What can I do ?

Answer :
Even though you are caught in a loop, there is a way to get out of this… but it involves a rather rigorous reset… to do so, do this :

• make sure you have the Synology Secure SignIn app on your iPhone (you can download it from the AppleStore – there are versions for iOS/iPhone and iPadOS/iPad)
• make sure you have SynologyAssistiant.app on your Mac (you can download it from the Synology Download Center – search based on the model of your NAS)
• find the tiny button marked RESET (on the back of your NAS) 
• use a paperclip, ballpoint-tip or something alike to press & hold the RESET button for 4 seconds, until you hear a beep
• immediately release the button when you hear the beep
• you have now reset the network settings of your NAS, removed the 2-factor authentication and reset the password to blank (but all data on your NAS will remain intact)
• now plug the UTP/Ethernet-cable from your NAS straight into your Mac
• temporarily switch off WiFi on your Mac (and disconnect all other network & internet connections)
• then, open the SynologyAssistant.app on your Mac
• when the screen opens, click on “Search”
• when your NAS appears in the list of found devices, click on “Connect”
• now your webbrowser (Safari) opens
• on the first screen, when your account name is asked, type :  admin
• then click the blue NEXT button
• on the second screen, leave the password field blank and click the blue NEXT button
• when prompted to enable 2-factor authentication, click “No, thanks”
• when prompted to set up adaptive MFA, click “I don’t want to secure my account”
• when the recommendation and question if you are sure to proceed appear, klick the blue OK button
• now the Reset Password page opens, set the account name you want to use (if you want, you can use your previous account name)
• now, if you click at the upper new password field, you will probably be recommended to use a Safari Strong Password, if so, click OK (if no password is suggested, set a long password manually)
• make sure you write down this password offline (or somewhere very secure)
• then, make sure the chosen password is the same in both fields and clcik OK
• when you get the “Success! Your password has been changed.” message, click the “Sign In Now” button
• then, you will need to log-in again on your NAS, this time with your new password
• if you get a message saying “password invalid” you can ignore it and try again (usually the second time it will just work anyway)
• when the webinterface opens, click on your account info (blue person icon) top right
• from the pulldown menu, choose “Personal”
• when the “Personal” settings window opens, click on the “Security” tab
• in the “Security” tab window that opens, select “2-Factor Authentication” and click the blue Apply button
• in the pop-up window that opens, choose “Verification code (OTP)” 
• then put your Synology NAS (a.k.a. “DSM”) account password in the next pop-up window and click the blue OK-button
• the next pop-up window advises to download the Secure SignIn app, but since you’ve already done so, just click the blue Next-button
• in the next pop-up window you will see a QR-code
• now, open the Secure SignIn app on your iPhone and click the OTP-button (bottom-right)
• when the OTP-window opens on your iPhone, click the + button top right
• then your camera will open within the Secure SignIn app
• now, using your iPhone, scan the QR-code that is displayed on your Mac 
• then, a 6-digit code will be generated on your iPhone
• key in that 6-digit code into the “Verification Code (OTP)” field on your Mac and click the blue Next-button
• in the “Backup email settings” window that opens (on your Mac) check that your account eMail address is also listed as your Backup eMail address, and verify your eMail address if you haven’t done that already
• if “Send verification email” is greyed out and “Verified” is indicated next to it in green, you can click the blue Next-button
• in the next window, click the blue Next-button
• now, your NAS has been successfully re-linked with the app, so you can close the “Personal” window by clicking the X button (top right inside the window)
• when done, in the online virtual desktop (which is still open on your Mac), click the Control Panel icon
• in the “Control Panel” window that opens, click “Regional Options” (in the “System” chapter)
• in the next window, make sure the “Time” tab is selected (usually the “Time” tab will be already displayed automatically)
• then make sure that “Synchronize with NTP Server” is selected in the “Time Setting” chapter 
• at “Server address:” select “time.google.com” and click the ‘Update Now’-button
• even if it says “Status: Checking…” or “cannot connect” you can now click the blue ‘Apply’-button
• Note : keep in mind that the codes generated by the Secure SignIn app are time-sensitive, so it is essential that the time set in your NAS is correct and keeps getting updated
• you can now close the NAS’ vritual desktop in your browser
• now, reconnect the UTP/Ethernet cables from your NAS and your Mac in the way they used to be before your problems occurred (so both your Mac and your NAS will have internet connection)
• then, on your Mac, reopen the SynologyAssistant.app
• maybe you need to click the ‘Search’-button, but ususally your NAS will already be listed
• select your NAS from the list and click the ‘Connect’-button
• now, your NAS’ login page will open in your browser
• on the first page key in your Synology NAS account name
• on the second page, key in your password and put a checkmark at “do not ask again on this device”
• on the thrird page, key in the 6-digit OTP code that is shown in the Secure SignIn app on your iPhone
• now your NAS’ virtual desktop will open in your browser
• there, click on the Control Panel icon
• open “Regional Options” again
• in the “Time” tab, check that the current time is displayed, that the ’Time Setting’ is on ’Syncronize with NTP Server’ and that ‘Status:’ is at “Normal” (in green)
• if not, adjust these settings and click ‘Update Now’
• else, close the NAS’ virtual desktop
• That’s it !

enjoy 😉