We've currently got the 2920 & AP 810 set up in wireless repeater mode so that as we move through our offices we don't have to connect to different SSID's
There is one area though that has a weak signal (and we can't move the AP) but it is sitting right next to our server cabinet so could connect via ethernet to the switch in there.
But I can't seem to be able setup the equivalent to a standard universal repeater (so using the same SSID as the router so we can move between offices on the same wireless) but with having the AP plugged into the switch rather than physically repeating the wireless signal.

Is that even possible (sorry i'm not a techy, well a bit)

If you want to plug the 810 into the wired LAN can you not just set it up in Access Point mode with the same SSID as your router? I have a 2860 upstairs, a 900 downstairs connected to the router by CAT5e cable and in wifi AP mode and a 710 out in the garage in repeater mode, all using the same SSID. Seems to work for me. I guess I can't always guarantee that a user device will connect to the nearest (strongest?) AP but I didn't want to go down the radius server route.

see here a guide if helps (is the same about with any model)
https://www.draytek.com/index.php?option=com_k2&view=item&id=2777&Itemid=293&lang=en