Lycamobile 'No Service' problem with iPhone 7

Hello, I own an iPhone 7 4.7'' and Lycamobile SIM card. Every time I get to the airport (in my case I travel between UK and Romania) and I switch from 'airplane mode' to normal mode, it takes a long time for the phone to find Lycamobile service (around 5-20 minutes). Usually I restart the phone or I mess with the network selection settings (thinking it is related to roaming settings etc.) to 'revive it' or something. But no matter what I do, after 5-20 minutes it finds signal... and I am always in places with signal... But why does it behave like this? Is it from Lycamobile? Is it from the phone? Has anyone experienced anything like this maybe with another network or with another phone?

Thank you,

Nick