Watch & Cellular Plan options

Greetings all!

I am curious about an issue I am running into with the cellular aspect of the Apple Watch. Specifically, the carrier and stand-alone plans. Here is an example. I recently moved back to T-Mobile as my carrier for a number of reasons, but I had no issue in porting my number back and getting set up on my iPhone 12 Pro Max. I added my Series 6 watch on their digits plan, and everything seems to be ok.

My wife is still using Google Fi as her provider on her iPhone 12 Pro Max. She has a Series 5 watch, also with cellular capability. What I have recently learned is that Google Fi does not support wearables. Further, due to what appears to be intentional design by Apple, you cannot use a different cellular provider with the watch if you are not using "the big 3" here in the US.

Despite this, T-Mobile offers a "stand-alone" data plan for wearables like the Apple Watch. So, we added service to her watch as a part of my account, but not on a "digits" line. It is a true stand-alone data/voice plan as far as I can tell.

The issue comes into play where in the Apple Watch app on her iPhone says because her carrier does not support a wearable (Google Fi) we cannot set up the data plan on her watch with T-Mobile. As far as I can tell, Apple does this intentionally, as the only other way to set up the watch would be to treat it as a device for "a family member" where the watch would then become an independent device, but then it would lose its ability to pair it with her phone and share activity data, etc.

So, my question is this... is there a way to achieve what we are trying to do? Which is to keep her phone on Google Fi, but have the watch with T-Mobile, or do I just have to move her back to T-Mo and then set up the watch on a digits line of service in order to be able to pair it with the phone properly?

Thank you, I would appreciate any constructive advice on this topic, particularly if there is a work-around to get the Apple Watch app to allow the cellular portion of the watch to set up on a carrier different from her phone.

Thanks in advance!