Reset Your Network Settings
Sometimes the issue can be a problem with the network settings on your iPhone. These can affect which towers your iPhone connects to and which cellular frequencies and channels it prefers to use.
Resetting your network settings will clear everything out, refreshing the default settings from your carrier.
You can do this by going Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings.
Just be aware before you do this that you’ll also lose all saved Wi-Fi passwords, connections to Bluetooth devices, and VPN connections.