iPhone won’t let go of WiFi fast enough

I’ve got a WiFi in my studio. After i leave the place, the WiFi signal naturally drops dramatically but the iPhone keeps hanging to it although on the same time I’ve got a full speed cellular 4G connection with perfect connectivity.

I’ve toggled the WiFi assist on, but I see no difference.

My iPhone drops the poor WiFi and changes into cellular after like 30 secs but that is way too much if I’m driving or walking away from the WiFi and the data transfer is completely stopped for that time.

Any help?

greetings from Finland, Europe