There are probably a number of answers to this, but with the given information you probably should consider connecting the WAN port to the LAN port of the office, treating the connection as a Router (WAN Settings=> Cable=>DHCP) and use a VPN service to connect to the outside world.
When possible use the LAN connection, but you will also have the option to have your own (WLAN) AP. If you want to tighten restrictions on your WLAN you can use the OpenWrt LUCI tools to set up the MAC filter to only allow your devices on your WLAN.
The GL.inet firmware should do this pretty much out of the box.