LB1120 Disconnect

I live in a very rural area and I’m unable to get internet normal. I got this modem and a booster and it worked great. But ATT has been giving me trouble and I decided to try a different carrier. Bought a bring your own phone plan from straight talk and set it up on an old phone. This connection works fine on the phone. But it won’t connect when I plug it in to the modem. I’ve tried resetting and factory resetting but there’s no change what did I do wrong????