Question about ASUS WL-Router RT-BE88U

I couldn't find a button, but it is possible to manage the entire network using an application on the assus phone (disable wifi, create a guest wifi, etc.).

Yes, you can use the WPS button. With a computer (Default web 192.168.50.1) Administration - System - WPS button behaviour instead of WPS enable for WLAN On/Off, and don't forget to click Apply.
And then press the WPS button for 2 to 3 seconds to switch WLAN on or off

...easy per Asus Router App. Aktivieren / Deaktivieren

Yes

Checked the manual again - no, unlike other models, this one does not have a button to switch off the WLAN. But as already mentioned, it is easy to switch off via the app or web interface. It is also possible to switch off the WLAN automatically overnight with a "scheduler", where you can set the time of day when it should be switched off.