Cellular

Does it happen everywhere, in all areas?

Have you talked to the carrier that gave you that SIM card? What did they say?

Have you tried to reset network settings? Settings>General>Reset>Reset Network Settings, keeping in mind that that procedure also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before, but NO other data is affected.

When done, restart your iPhone, then test it.

If the issue still persists, have your device inspected at an Apple Store or Authorized Service Provider.

Regards

Giulio

If your device is a model 6s, it does have LTE connection capability.

Choose your country and check if your cellular carrier supports LTE:--> Wireless carrier support and features for iPhone in the United States and Canada - Apple Support

Regards

Giulio

I have found that if I start the phone in a place with strong 3G and slightly weaker 4G/LTE signal, the phone will lock onto the 3G signal and ignore the 4G signal. Sometimes it takes as many as ten restarts to get the phone to happen upon the 4G signal before it sees the 3G signal, after which it will have no problem remembering that it likes 4G. Good luck.