http://airvpn3epnw2fnsbx5x2ppzjs6vxtdarldas7wjyqvhscj7x43fxylqd.onion/forums/topic/18998-how-to-airvpn-on-synology-dsm6-and-dsm7-complete-guide/page/3
You need to enable port forward on your router, forward those ports, and know your REAL ISP IP. For ISP IP, it's easier to use Dynamic DNS. Asus routers provide their own free DDNS sevice for example. When you setup DDNS and port forwarding, and finish step 3., then you can access your Synology even when AirVPN is running, for example: https://mydomain.com:5001 for GUI or https://mydomain.com:6690 for Drive/Cloud station...