Phone is 4G but showing 3G

You cannot do it yourself. That is entirely up to your carrier.

If it is connecting to a 3G network at your current location, that means there is no LTE/4G service available there.

Yes, it is an LTE / 4G capable phone.

Your carrier needs to be providing 4G service in your area and you need a compatible SIM.

I don’t know where you’re at, but in most of the world, 3G networks no longer exist or are being phased out.