Cannot log in using old iPad

@dmercer:

Understood, I thought you were already inside the app, and trying to add your account from there. Even if we cannot check it from the app itself, I think this might be happening because of the fact that it's an old version of iOS with an old version of the 1Password app. For security reasons, we don't allow old clients to connect, and I think this is what is happening to you.

If you look at the https://support.1password.com/system-requirements/ for 1Password for iOS, you can see that 1Password version 7 is required to use 1Password accounts in the app. This, in turn, requires iOS 12.2, but if you are running iOS 10.3.3 (like you wrote in your first post), the maximum version of the 1Password app that you can install is version 6, which does not allow you to add 1Password accounts.