Reestablishing connection to a credit card issued by a bank
I posted this elsewhere as a comment but want it to be more visible.
When you login to your banks' websites, does the "show my credit card details" take you to another website that requires a login? In my case, Citizens Bank's credit card is handled via the "access my card online" website with a "brand" designator. Quicken does not have a mechanism that I can find to change the financial institution to the one that actually hosts the credit card. It always reverts to the issuing bank even after removing the account's online connection (deactivate/reactivate).
Contacting the phone number on the credit card usually gets you to the company that hosts the credit card and you can ask if they stopped supporting Quicken. They probably don't know. Regardless, try the procedure I show below and good luck!
To reestablish connection to bank issued credit card, in my case Citizens Bank credit card, look for the file fidir.txt, probably in C:\ProgramData\Quicken\Inet\Common\Localweb\Banklist\2019 . Open it with Notepad and search for accessmycardonline.com
There may be several hits, select the one that matches (brand name) what you see for a URL when you login to credit card. In my case that was "Citizens Bank - CC" (product=CITIZENSBRAND).
Add a new account, and for financial institution enter the one that had your "brand name" ("Citizens Bank - CC"). Use your website credentials and you may get prompted multiple times for secondary security (challenge phrase or verification code) via text or email. Once the site allows access, link your found account(s) to the already existing account in QuickBooks. Recent data should be downloaded.