Q: Why does my iphone 6s not connect to 4G?

Tap Settings > General > Cellular. Make sure Cellular Data is turned ON.

If it's already turned on, try this support article >  If you can't connect to a cellular network or cellular data - Apple Support

Hi

Go to settings mobile data look at mobile data options Enable 4G

You need to be in a 4G area or it will use 3G if not .

Cheers

Brian

Where did you get the phone? What does it say in Settings>General>About>Model? What country are you in and what carrier are you using?

Different models are sold in different regions as not all carriers use the same LTE bands. If you have a phone sold for use in a different country/region, it may not work on LTE service in your country or your carrier. If you are using an unsupported carrier, you may not be able to get LTE service at all.