1. Upgrade to 3.x firmware if you are using v2.0
  2. Set up openvpn or wireguard server on the router. I am not sure why you say wireguard is not working.
  3. Set up port forward on your main router, e.g. 8080 to 80 on the mini routers ip (e.g. 192.168.1.xxx)
  4. Access the mini router using your public IP, xx.xx.xxx.xxx:8080