Is the issue occurring where you (and other phones) would usually get LTE signal? I did have that issue with my iPhone5, so I reset the network settings & changed the SIM card but the problem persisted until I performed a factory reset on the phone.
I've had the same issue ever since I changed from the 5s to the SE. I had NEVER had issues before. And this issue occurred only in my home with the SE, even though the 5s always had LTE. I just spent a few hours troubleshooting, and finally found the fix. All I had to do was reset the network settings!
1. Open the Settings app.
2. Scroll down and tap the “General” menu option.
3. Scroll to the bottom and tap the “Reset” menu option.
4. Choose “Reset Network Settings” in the middle of the screen.
5. Enter your iPhone or iPad’s pass code to authorize the reset.
6. Tap “Reset Network Settings” to confirm the reset.