Ewon device will not connect to internet

We have an Ewon Flexy 201/205 that will not connect to the internet. It requires 4G, I believe it uses PPP and works fine with a T-Mobile sim straight out of a phone but not with two different AT&T sims. All three sims work in an AT&T mobile phone but not in the EWON. Pretty sure it is something in the AT&T network that is blocking the connection, or a setting that we need to get AT&T to make but have run out of ideas/options.

The Ewon is a commercial modem used inside an electrical cabinet.

Any ideas appreciated. Thx.