We have a machine with Draytek Smart Monitor. It monitors all traffic going through port 2 on the router, but if people connect to the router via the built in Wifi, they are not picked up in the Smart Monitor software. Is there any way we can mirror the wifi port as well? We can't see any option on the draytek gui so maybe a telnet command?

As far as I know you can't, the monitor port stuff is a function of the switch in the 2820, the wireless isn't counted as part of that switch.
Personally I just use a separate access point on the LAN side.