Hmm.. not sure.

I've since removed the O2 Router, so now I'm using the 2820 as router (with wireless on, DHCP on, 192.168.88.1)

Just tested the 2830's wireless (default SSID "DrayTek") and it works for me. (2830 on 192.168.88.0, DHCP=off / relay agent to 192.168.88.1)
- I can (wirelessly) access the internet, access both admin pages of the 2820 (192.168.88.1) and 2830 (192.168.88.0)...

Unless anybody else have a better idea, I would suggest to do a factory reset and try again.
( - I've not got my VLAN to work at all... must be something buggy on this first firmware...)