ATT nighthawk M1 mr1100 no sim installed

I have an AT&T netgear nighthawk m1 mr1100 that has been unlocked. I am on AT&T session based prepaid plan. I have been using device and plan for nearly two months when my device started saying sim not installed. I went ot store and purchased two more sim cards thinking the sim card became corrupt somehow. I get same error with of my ATT sim cards. I have tried other brand sim card to see if they are recognized in device and Sprint/Republic Wireless as well as Mint Mobile sim cards will show broadband disconnected which tels me the device is not working with ATT network sim. Any suggestions? ATT support is not helpful, they only say to try a new sim, but I did and it still doesn't recognize.