After the hardware reset I changed the AP to my liking. Then I configured the STA to connect to my home WIFI.
Subsequently, I have set up the Wireguard VPN.
This was immediately active.
Since I want to use the router in my caravan, I have to constantly change the WIFI network when traveling.
After changing the STA, the VPN configuration was not displayed under Status > Wireguard and no VPN was established.