4G signals are weak where I live, and iOS always falls back to 3G. To manually switch to 4G, go to settings, select mobile data, then go to mobile data options, then tap voice and data, switch to 2G for a few seconds,then switch back to 4G. Now you should be on 4G network. ?
My iPhone very often has unstable connections. It’s connected to the WiFi but has no internet access. I’ve tried resetting the network, rebooting, and forgetting the network but it doesn’t work. Then I found there seems to be an easier option: just enter wifi settings and hit the blue i icon and then you’ll see the following interface. Simply hit the renew lease button on the bottom and voilà! You’re online again!