My T-Mobile LG G2 been having same problem for a while, I access hidden menu in phone dial app with 3845#*801# and select LTE-Only > Modem Settings . Now there are 2 settings in here worth checking out. First is RAT Selection where I can choose LTE/GSM. Second is LTE Band Seclection, where band 4 is T-Mobile LTE, band 2 is 2G or 3G I think.
As soon as I ticked the check for band 4, I got data 4G back. But it revert back to 2G or that error message once a day probably when I get to area with low signal. From B. Diddy suggestion, I just now tried select LTE/GSM option. And the service does come back. Hopefully with both settings correctly selected, I won't lose signal anymore. Will report back in a couple days. Thanks and good luck !
update: Doesn't permanently fix it, still revert back to 2G ~once a day. Have to go to hidden menu and recheck the boxes to get back 4G LTE
