move Home Internet SIM to personal router with external antenna

When I was deciding on a home internet solution 6 months ago, I inquired at the TM store about using my own router because I thought I might need an external antenna and the TM router didn't have a port for one.  At the time Nextgear sold a 4G router that supported an external antenna and was claimed to be compatible with T Mobile.  The rep at the store wasn't sure if it would work,but was willing for me to "try a SIM card for an iPad".  I ended up not doing that, and just went with TM's own hardware (signal reception ended up being quite good)….but it sounded like if you want to use your own router there may be restrictions on how the SIM card is configured.