You used to be able to use the ##4636## trick to get a hidden menu, but now that’s gone too. There’s nothing in the menu that alllows you to select which network to use. I hate to keep having network disconnects and in perpetual 4G/5G back and forth and battery draining.
I use Samsung btw
Edit: For context: Its due to issues with being near the edge of the 5G zone so your phone keeps going back and forth between 4G and 5G which (I suspect), is what causes constant network disconnection, phone being hot/warm, and battery drain. Very annoying.
Settings -> Connections -> Mobile Networks -> Network Mode
Works on my S23 Ultra, not sure if they lock this down by region or vendor though.
Not on mine, some carriers apparantly can make that option disappear somehow. (Idk why they want to do that, their 5G coverage is so bad)