Nighthawk MR1100 M1 - Inserting iPhone ATT SIM Card

I've searched a bit without many answers.  Is ATT blocking SIM cards for phones that are placed into the M1?  

I was looking at a used one and the person took their iPhone SIM placed into the M1 and it worked.  I'm assuming it takes ATT some time to cycle through.

When I added my 5th phone line, I never gave ATT my phone IMEI.  They simply sent into to me in the mail and I stuck it in my phone.  Wondering if I can use that 5th line in an M1.