[Q] Your sim card doesnt allow a connection to this network. wtf. topic

Rrecently bought my 3rd opo. So this one will not let me hse my att sim. It keeps saying "sim card does not allow a connection to this network" wtfff. Im very sure the sim is in right. Contacts facing the back. Im using a unlocked lg g2 with the same sim and it works fine. I domt know what the heck. The "no sim" icon dissapears when i insert the sim. Jut no service at all.HELP.

Att user
Sim works in other phones.