SIM card issue for 11" 2018 iPad Pro

When I first got my 11" iPad pro in 2018 (WiFi/Cellular), I added a line of cellular service for it on my AT&T family plan. I also have an older iPad Mini (WiFI/Cellular) for which had a SIM card connected to my AT&T family plan.

After about a year, I took off the cellular plan for the iPad Pro because I was barely using the device.

Fast forward to 2020, and it would be useful for me to be able to occasionally have the iPad Pro on cellular. I thought that it would be as simple as taking the active SIM card out of my iPad Mini and inserting it into the iPad Pro, but that seems to not be the case. It doesn't see the SIM card - in cellular settings it just tells me that the older plan I had in 2018 (with the built-in SIM) has expired.

I have a horrible feeling that this is simply a non-sequitur of the way Apple has set up iOS, to default to the status of the built-in SIM if it's been activated at any time.

I'm obviously trying to avoid having any contact with AT&T for a few reasons, not least is that I don't want to change the line of service permanently to the iPad Pro, secondly they charge a fee for doing so.

Any suggestions?

Thanks