You will need to contact your carrier for assistance with network issues, if your phone was not designed for your carrier, that might be causing issues, and that is not something you can "fix", it would be a hardware limitation.
Did a post have a solution that worked for you? Help other people find solutions faster by marking posts that helped you as an "Accepted Solution". Learn about accepted solutions: Learn More.
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.