LB1120 not accepting sim card

I reset the modem to factory defaults (as described in the previious reply)

After reset, the router still does not recognize the T-Mobile, activated (and well-used) data SIM. I have an AT&T data SIM that works just fine in the router. It is the T-Mobile SIM that the modem cannot recognize. 3rd green bar steady, blinking amber.

that data T-Mobile SIM works fine in an Alcatel Linkzone hotspot. The router was purchased to upgrade the hotspot. (that is, the location has T-Mobile signal)

The AT&T SIM works fine in its AT&T hotspot as well as in the LB1120

So, AT&T SIM: recognized by LB1120

T-Mobile SIM: NOT recognized by LB1120

The LB1120 is fresh out of the box. It has not been placed into service, though I did check that it passed data through the working AT&T SIM.

Any ideas?

to be clear, I can always make a mistake.

thank you for your help

background:

I've been in high-tech for nearly 40 years. I've authored multiple books on computer security. I regularly guest lecture at universities about computer science. I've swapped SIMs successfully for years across a variety of devices. At my 2nd high-tech job (a long time ago, in a galaxy far, far away), I went to each customer to add memory to their motherboard by socketing additional chips. Seating a SIM card seems a good deal less scary then bending a chip pin. 

So, I'm giong to take a wild guess that the problem is probably NOT me (but I'm open to suggestion).