Whenever you accept the TOS, your device is somehow registered/authenticated against their servers. Such a session establishment of course should be secured through TLS, just like all web traffic in general. Frankly, I see the issue here clearly on your side; you have to make sure your device supports up-to-date cryptography standards.
I saw in a different comment that you do not want to replace your phone but you definitely have to replace your software. Find an older build of lineageOS (well, probably even CyanogenMod in this case) and migrate to that. Even if it is based on Android 8, it would still be much more in line with modern security than what you are running now.
Btw, the complaint of you not being able to do banking through your browser anymore while it does not support TLS 1.3 really made me laugh, thank you!
I don't think you realize just how big the risk is that you are putting yourself in with such old software.