I want to use Air VPN to establish remote access to my Raspberry PI 2b+ from internet.
This is not what AirVPN is here for. If you want remote access you'd install an OpenVPN server on the RasPi and connect to it from anywhere you want. If you need help with this, someone around here can surely offer help.
But even if you want to use AirVPN, there is a way: Install an OpenVPN client on the RasPi, connect to AirVPN with it, then forward the port you wish to access your RasPi on on the port forwarding page in the client area (don't forward on your router, please) and while you do that, set a DDNS name so it's easier to access your Pi. If you want to do this instead, just say the word and someone will come up with a guide for you to follow.
I'd recommend going through Mr. LZ1's New User guide for links to some guides.
Thanks for reply.
But will I have any limitations using AirVPN for this purpose?
AirVPN allow to forward ports, connection will be encrypted, I can connect up to 5 devices.
So I think that's a good way to connect RasPi in this way, am I correct?
And what about IP address, it will be stable if I always connect to one server?