FYI. Since Starlink is taking longer than expected and my mobley has been needed to be reset more and more frequently I went ahead and decided to upgrade my LTE modem.
This weekend I bought a Netgear Nighthawk Mobile LTE modem from Amazon for $225. It arrived the next day.
My SIM was locked because of the v1.3 upgrade on my Mobley so I called AT&T support and requested my PUK1 & PUK2 codes for my SIM and that worked fine. They will still hand it out.
I swapped the SIM and the new device is getting 30mbit when the mobley only did 3. Now I have an external LTE antenna for the Mobley that gets it to rates at 20-30 mbit, but the Nighthawk uses a different antenna connector so it will be a couple of days before I can try the Nighthawk with the big antenna.
So just saying it is still possible to move a SIM. The Nighthawk allows 20 devices by default, but if you disable the internal routing features you can use an external router then you don't have any limits on the number of possible devices. After a day of use, it seems stable.