2.4Ghz Wireless, DHCP not working, but fine on Lan and 5Ghz

Fresh install on D-Link DIR-885L as of yesterday, and having a strange problem where the DHCP server doesn't seem to respond to requests on the 2.4GHZ Wireless interface.

Connecting to the Lan or 5Ghz is fine. The connection is working as seem to be given an IPV6 address, just nothing on IPV4.

I've checked the logs, and there's nothing obvious, and tried rebooting, reconnecting the wireless device to no avail.

The really odd one is my raspberry PI (configured to connect to the 2.4Ghz wifi) - it doesn't get an IP for the wireless, but if i connect to the LAN as well, both interfaces get an IP, but can't connect via the Wireless (and the DNS only sees an entry for LAN not one for the Wireless).

Suggestions as to where to investigate next would be appreciated - it's probably something simple like a duplicate IP from when i was configuring it all, but i can't spot it.