To run as VPN client or server, your Mango will have to be in WISP repeater (default) mode. It will not work in AP mode. You will end up with two IP subnets.
Thanks for the advice limbot, presumably I can use repeater mode whilst the mango is connected to the LAN? When reading the instructions, it seems that repeater mode is used to connect to an existing Wi-Fi network, and not via the LAN. Thanks.