@DaWeav Thank you for your response, much appreciated 
So, that means adding your own device and calling it 'br-guest'.
I was originally thinking of doing that, but wasn't sure if that would break any of the proprietary parts of the GL.iNet software (ie: their own updates made to OpenWrt 21.02 that comes installed as factory default).
However, if this is the only way to go then I will just add an additional vlan (on br-lan) and assign that as the device on the guest interface.
I was thinking of updating to OP24 (as my DAP is running stock Openwrt v24.10), but having read [4.7.5-OP24] Known Bugs I'm not sure now is the time. Maybe wait a few more months and hope some of those bugs are cleared out!?
Once again, thank you for your help