Hello. I bought an LG Optimus F3Q for my wife because she prefers the slide out keyboard. I checked that it is compatible with T-Mobile 4G first. The phone does receive a good 4G signal, however what happens is that if it doesn't receive a good 4G signal, it will drop to 2G. That's fine, but it will not go back to 4G when a strong 4G signal is available again. The only way I can get it back to 4G is to search networks and choose an AT&T signal then search again and choose T-Mobile RAT:LTE. When searching, available networks are T-Mobile LTE and GSM; AT&T LTE, WCDMA and GSM; Verizon LTE. When searching networks with my Alcatel phone, it sees T-Mobile 4G, 3G and 2G; AT&T 4G, 3G and 2G; Verizon 4G. So the LG doesn't see the T-Mobile 3G signal. I've read that if a phone is on 2G (GSM) and it doesn't see a 3G (WCDMA) signal it won't look for a 4G (LTE) signal. Is that why the phone doesn't go back to 4G? I've tried all the preferred network type options by dialing *#*#4636#*#*, but none will make 2G go back to 4G. I can force it to 4G only but then I get no signal at all if 4G isn't strong enough (like in the house). Any suggestions?
According to LG Optimus F3Q - Full phone specifications, that device should get 3G in many areas (any "network modernized" areas that they have some 3G deployed on the 1900 frequency). I'm guessing if you're not seeing 3G on that device, then you're not in one of those areas.
If the phone is programmed to not try to go back up if it doesn't get a 3G signal, I don't think there's anything that can be done. You could contact the manufacturer to see if they have any suggestions.