At home I have a 2830 downstairs and in the attic, I think, is a 2925

The 2925 is wired to the 2830 and they both have the same wireless details for connections.

How do I set these routers up so the WiFi goes off at 11pm every night?

Do I need to do the same to both?

I have tried using the schedule function but it doesn't seem to work so I'm not doing it properly.

on my 2925 it looks like:

Wireless LAN | General Setup

Follow the link to create 1 schedule with the action being "force down" for the overnight no-wifi period

then assign that profile number in the Wireless LAN | General Setup schedule box (of which there are 4)

I've tried that but nothing happens.

I set the forcedown action in the schedules.

I only use SSID1 so put 1 in the first box.

Nothing happens. WiFi is still up.

This is where I've been stuck with this. I can't work out how to make the scheduled force down work.

Could you try the actions in this guide to see whether that allows your wireless scheduling to work?
https://www.draytek.co.uk/support/guides/kb-wireless-lan-scheduling