Can't access Raspberry Pi (or internet) with Pi-hole as DHCP Server

Yeah, you're entirely correct @jfb . Same with you @DL6ER.

Must have had a brain fart, I don't actually remember restarting the router. I also followed the guide here which doesn't specify restarting the router, so it may be worth adding that step.

Thanks for the prompt help guys, I'll give that a crack when I have some time to play with.