Well, it turns out it was already working last night - kind of. I switched it off after some tinkering and went to bed, fired up again this morning and ta-da! And of course it would, it obviously needs a reboot after turning DHCP off 😑 So yeah, it was set but it never had a chance to pick up its new IP from the mothership because I'm a plonker I guess. I can connect to its wifi and cast to my wired Apple TV in the other room etc, so it's all talking nicely to each other.
Only weird thing is, I can see it when logged as admin on the main router and see its IP, but when I try to go to the IP to look at settings/change wifi name etc it doesn't connect, but that could be easily circumnavigated by taking it off the network and wiring in directly with the laptop I suppose, which I will try after work.