(perfectly working) android phone that I had had for 6 years. The phone brand was not providing android updates for it.

I get it through from the banks perspective. The OS is no longer getting security updates, so this is a fault of the phone manufacter for not having a long term commitment to security and OS updates.

And the app itself required a code that I could only get by SMS.

Yeah this is pretty bad.

And I will be stuck with them for decades.

Why's that? I have my mortage through one bank but literally never use the actual services of the bank, I've never called them or used their app or logged into the site. My current account and savings accounts are with another bank separate to my mortage provider.

Also, won't you be able to change when you come to re-mortgage if another bank offers a better deal?