FP2 has no setting to disable network mode fallback, meaning there is no 3G only, 4G only or 4G/3G only mode. My phone did often fall back to 2G although a (weak) 3G or 4G signal was still available. And a weak 3G or 4G signal will typically give you better performance than a strong 2G signal.
There are special apps and Xposed modules to achieve this, but I was looking for a more direct approach. You will only need root and a terminal emulator app (or ADB).
Assuming SIM2 is set to 2G and you want to set SIM1 to 4G/3G only (no 2G fallback):
settings put global preferred_network_mode1 12
settings put global preferred_network_mode 12,1
Reboot and you’re done.
(For 4G only use
11, for 3G only use
2 instead of