G920R4 Unlocked but weak reception

After unlocked from Z3X and setup APN by Shortcut master, I’m able to use GSM sim card like ATT or Tmobile and get 4G LTE.

But the problem is the phone got really weak reception when the 4G LTE icon appear. Most of the time the signal bars become gray and I’m unable to send SMS. But I still can make a call.

Maybe the phone check signal bar before send SMS, when signal bars is gray then the phone understands that there is no signal so it refuse to send SMS.

Is there anyone knows how to fix this problem?

Thanks for your time