@kevincork Don't use DHCP for those devices. Instead, assign them static IPs and whichever DNS server you'd like.
Also, like @jfb said, don't use a secondary/alternate DNS server on your router. Otherwise, your ad blocking will only appear to work some of the time. Instead, tell Pi-hole to forward DNS requests to multiple DNS servers, such as 1.1.1.1 and 1.0.0.1. Pi-hole will filter out all of the ad server requests while sending the non-ad requests to Cloudflare's DNS for normal resolution.