Can't use mobile data after having enabled WiFi (unless I restart my phone)

[JSleek]

Hi there,

I'm having this annoying problem with my LG G4 (Android 5.1, software V10a-EUR) where the phone will not connect to mobile data AFTER I have had WiFi enabled for a while.

The only way to get mobile data to turn on at this point is to restart the phone, at which point when I enable mobile data it will turn on.

My carrier is TalkTalk, I'm living in the UK.

Any ideas on how to fix this?

All help is very much appreciated. Cheers!