I also just tried to set up C24 (German banking app) and have a similar experience. It should be considered an app that does not work on /e/OS (at least current 4.0+ /e/OS).
I would potentially be able to use a non-/e/OS device in the household to finish the account registration (C24 is so infatuated with the app age that it doesn’t even let you finish account registration on desktop!). But then for every login afterwards I’d probably run into the problem @D529 described, which means I’d have to use a non-/e/OS device daily to use the app with my /e/OS device, which ultimately invalidates a “working” status.
The problem in detail, as new account creators experience it, for documentation’s sake: I installed the C24 banking app from App Lounge, opened it, put in my phone no. and the SMS code, put in an app PIN and after that I get a “Sicherheitsüberprüfung fehlgeschlagen” (security check failed) window in the app. The linked article in C24’s help center only describes in general that they just support official Android, no root, from official app store and so on. My device is not rooted, bootloader is not unlocked, USB debugging is not on.
I tried this workaround, but this already fails because in Shelter in the Shelter menu there is no “microG Services” entry: [LIST] Banking Apps on /e/OS - #481 by AudeM