Can't deregister old iPhone 7+ number on new iPhone Xs.

I followed the steps above, basically changing my Apple ID password. But the number is still listed in both my iPhone and my iPad. I no longer have that number, as I've switched carriers and numbers, even after shutting down and restarting both the phone and the iPad.

When I click on Edit in Settings>Phone>My Name (Apple ID)>Name, Phone Numbers, Email>Reachable At, I get this message:

I then click on Learn More, and get this:

I then enter the number I want to delete (my old phone number) but I never get a code at my new number. When I click on Didn't receive a code? it takes me to a page than then refers me right back to this page!

This process is absolutely ridiculous! It shouldn't be so difficult to remove a number that is no longer in service. It's actually infuriating! I want my old number removed from my Apple ID or whatever needs to happen immediately, as the number will soon get given out to a new customer at AT&T, my old phone provider.

What do I need to do to make this happen? As your online instructions clearly don't work!

Thanks

I called Apple Support today and they were unable to help. The first guy seemed to understand the problem, but was unable to fix it as the tool to remove the number (which comes up as challenged) was no longer available to him. He then transferred me to another, more senior tech guy who said I need to talk to T-Mobile because I have a dual SIM card. I called T-Mobile and they said that wasn't the problem and that I need to go to an Apple Store, of which there are none close by. This is very frustrating and is really turning me off Apple products, in general.