iPhone 7 (iOS 12) - Slow Internet Connection on LTE

Hi,

It's been 6 months since I bought this iPhone 7 A1660 in Indonesia. Currently I'm facing a problem when I'm connected to the LTE connection, the signal bar is full however there's no internet connectivity at all.

I've tried to reboot, reset, and even factory reset the phone with iTunes, still not working. The issue is only when I'm using LTE connection. When I switched back to 3G, everything is fine.

Could someone help me to solve the problem? I've contact the carrier and my SIM had replaced with a new one. Still not solving my problem. My wife also using iPhone 7 A1778, I tried to switch the SIM card, mine works perfectly on her phone. But her doesn't.

Thanks before.