App synchronisation troubles

Hey everyone !
I’m new here, got my Z18 a few days ago but I have some troubles with all my apps. (Latest version of OS)

It doesn’t sync in background. I mean, regurarly, during the day, I have to open WhatsApp, Messenger, Telegram and even SMS/MMS to get the new messages and the notifications. It looks like the phone is not looking for new messages.

I tried to restart the OS, looked into the settings, but I didn’t find any solution.
Does anyone has the same problem ?