Android wifi ok, then suddenly disconnects with "authentication problem"

[AC Question]

Hi,
I've got an Android MyPhone L-Line cell phone and as the title says I can connect to my wifi without problem. But after some time when I check the phone I'm on 4G and in wifi settings it says "authentication problem". When I tap on my wifi name and tap CONNECT, phone connects just fine ... for some time.

I checked for firmware update and my phone is up to date.

Thanks for ideas!