Hi al,
https://openvpn.net/vpn-server-resources/openvpn-access-server-installation-options/
https://openvpn.net/vpn-software-packages/ubuntu/
To access it from outside (remember, even if your phone on cell data is in your hand in your home, it is "outside" your network!) you must configure your router to forward these ports to the Access Server VM:
- TCP port 443
- UDP port 1194
Note that in your case this also implies that the router also knows how to reach the Virtualbox network, and that your Windows machine with Virtualbox is properly forwarding packets between your LAN and your VM.
You might also be interested in trying the
OpenVPN Cloud service, which will not have as steep a learning curve as trying to learn both Linux and OpenVPN at the same time. 3 connections to your Cloud are free, as opposed to 2 connections for Access Server.
Cloud has a separate forum here.
regards, rob0