RaspberryPi Access Point Configuration + Bootup Options

Hi, I need to extend the possibilities of my Pi as an Access Point. (Ethernet and LTE-Stick) What I want: -Automatic decision on bootup: Is Ethernet/LTE-Stick/nothing available -> Forward Internet from Ethernet to WiFi/ Forward Internet from LTE-Stick to WiFi/ Just create WiFi What should work in all cases: Raspberry should be available via SSH and/or VNC over the WiFi the RasPi is creating. (If Pi has Internet from Eth or LTE accessing it over the internet with RealVNC is a piece of cake) - After that: show me what you have done in a little HowTo document What is already working: - Raspi is configured as a Access Point, converting Internet from Ethernet to WiFi. - i configured up the LTE-Stick and the Pi was connected to internet. (need to be setup again) - RealVNC is working when connected to Internet ( But doesn't work without)