In Lg g pro f240 l/s/k many people find problems when their phone automatically starts switching from 3G to 2G and same for 4G. They are not able to find any option of changing network mode i:e from GSM(2G) to WCDMA(3G) or to LTE.
So here is the solution and a simple one
Change Network mode 2G 3G 4G GSM WCDMA in LG G PRO F240 L,S,K
Open dial pad and dial 5457#*240#
See screen shot
You will see this menu scroll down and select network mode
Now select WCDMA for 3G or LTE which one you prefer .
The phone will restart and your problem will be solved .. You can always come back to auto mode by doing same procedure.