Putting a Family Watch on a Different Cellular Data Carrier than the Data Plan/Carrier of Your iPhone

ISSUE

I have gone through this process twice and there is a bug/design issue where the iPhone will try to automatically set up the cellular data for a Family Watch to the same carrier as the iPhone.

So for me, my son had an Apple Watch with a T-Mobile cellular data plan. However, my iPhone is on AT&T. Every time I tried to setup the Apple Watch for him it would default to AT&T and error out.

Here are some screenshots of what was happening:

Both times I contacted Apple and they would deep-dive to diagnose the Watch to no prevail. The carriers would send me back to Apple and it was an endless cycle.

The SOLUTION: I went to T-Mobile and basically had them setup the Cellular Data plan specifically for the IMEI of the watch. I had to go into the store to do this and they had their own run around because they typically like to send a code to the device (yadda, yadda). However, once they had confirmed the Cellular was connected to the device and I went back through the Cellular Data setup for the watch on my phone, it immediately connected to T-Mobile instead of trying to route to the data provider on the phone.

I'm posting this here because Apple Support and Phone Carriers all seem to struggle with this and I'm hoping to save someone the time and effort of dealing with this themselves.

[Edited by Moderator]