The AP900 leaflet says that the USB socket can be used to support a printer. I've tried this and the printer is not recognised in any way on the LAN. Anyone know how to make use of this port for printing?

Complete guess - but I suspect you add it as a 'Network Printer' - specifying the AP900 as the host and the port to use as 9100 (LPR printer). Windows 10's "Add a printer" wizard might just do this automatically -you never know your luck!

See: https://www.draytek.co.uk/support/guides/kb-vigor-lpr-setup#windows-10-8 ... though it's talking about Routers rather than Access Points, it might help.

The user guide for the AP900 does indeed indicate that the USB port can be used for a printer.

I note however that later models (AP903) say that the USB port is just for the thermometer accessory.

I seem to recall that on the routers one had to enable Bonjour LPR Printing in order to make a printer visible.

That doesn't appear to be an option with the AP's....