Network disconnecting after returning from flight mode Device: LG G4 with android 6.0

When I turn flight mode off, after a few seconds I get reconnected to my mobile carrier network. After a another few seconds though, the network disappears (showing 'searching for network' icon in notification bar) and I have to wait for a minute or so in order to be able to make a phone call or something. 

The phone came with android 5.1.0 and I think this started happening after the first OS update (to 5.1.1 if I can remember correctly).

Why is this happening? Is there any way to fix it?
For some reason this had to do with my service provider. When I activated 4g the issue was resolved! (don't know why and how this is related to 4g activation but it really stopped when I called the company and told them to activate my 4g)