Wi-Fi switches off randomly?

Following problem.

My Wi-Fi actually works fine. When I have activated the mobile phone and use it.

As soon as I switch the mobile phone to standby, it comes! Sometimes! Before that the WLAN was switched off almost all the time. At first I only see my four telephone network bars but no WLAN symbol and that comes back a second later… It's sooo infinitely frustrating when you wait for a message and it went on all day and then exactly at that moment…

Why is that sometimes?

Mobile phone: Huawei Mate 10 Pro (always on display activated)

Possibly. Power saving mode?

No not really