OnePlus
Set Preferred Network Type (2G/3G/4G) auf OnePlus
The preferred network type tells your OnePlus modem which mobile network generations it is allowed to search for. On Automatic settings the phone constantly compares 2G, 3G and 4G signal strength and switches between them, which is efficient in strong coverage but can cause repeated reconnects, choppy calls or slower data in weak reception areas.
Locking the device to a single mode stops that constant switching. Choosing 2G/3G (Automatic) skips 4G entirely and can help in areas where 4G signal is present but too weak to hold a stable connection. Choosing 2G only goes a step further and is mainly useful for basic voice calls when data speed does not matter, or to save battery when no higher network is available anyway.
How to change it
Open Settings ➔ tap Wi-Fi & network ➔ tap SIM & network ➔ tap Preferred network type ➔ choose 2G/3G/4G (Automatic), 2G/3G (Automatic) or 2G only.
Switch back to 2G/3G/4G (Automatic) once you leave the weak-coverage area, since a locked lower mode will cap your maximum possible speed even where 4G is fully available.

