Subnet for IoT with 2nd Wifi router on RPi 3


I’d like to use my RPi 3 as MQTT and Node-red Server. So far so good. But I want to move my iot devices into a seperate subnet. My thought was to open a Hotspot on the RPi and further install a VPN Server on it be able to access it from both main subnet and internet.

I turned off eth0 and get a static IP on wlan0 (built-in wifi) while wlan1 is interfacing a wifi dongle. Unfortunatly the DietPi WiFi Hotspot software refuses to work without eth0. Is it somehow possible to replace the inital check for eth0 with wlan0 to get the installer running? Or am I on a wrong way anyway? I tried to setup it manually and read almost any tutorial which I found in google, but I don’t get really forward with it.