How to Switch your iPhone to a Better Tower
To manually force your iPhone to switch cell towers:
Open the Settings app, Turn off WiFi, then select the Mobile tab. Next, select Mobile Data Options, then select the Voice & Data tab.
The Voice & Data setting will likely already be set to 4G, VoLTE On.
Wait 30 seconds, and then cycle it back to the previous setting, either 4G, VoLTE On or, 4G, VoLTE Off.
Once your iPhone's LTE antenna reconnects, it will search out the antenna with the strongest signal and connect to it, likely a different one that you were initially having issues with.
If it appears you're still connected to the same mobile tower, it likely means there isn't a better option nearby. Other towers may be farther away, have a weaker signal, might not be using compatible frequencies, or may be congested.