Signing in via the settings app will just not work. Instead, you have to do it in the App Store. But first, create an Apple ID account on another device and go to music.apple.com/. There, you have to accept the privacy policy.
Why do I have to do this? The App Store on the iPad can not display the privacy policy screen, so you have to do it on another device.
When you've done that, open the App Store on you iPad, scroll down on the Featured page and click Sign In. Then, input your Apple ID and password. It will say that it failed, but you'll receive a 2FA code via SMS or email. Open the Sign In prompt again, type in you AppleID and password, but in the password field, append your 2FA code. You should then be able to log in.
If you've done everything correctly, you should be able to download still supported iOS Apps :)
Click here to learn how to download Apps that are no longer listed on the App Store