No worries. Under the DHCP tab that you showed is there anything below that area that says "customer option 1" looking for anything dns related.
Also, a few tabs over from the "LAN" tab there is a Manual DNS tab. Is that the same tab as the WAN tab you first posted?
Added comments:
I did some searching on the web with little help. The online manuals I've found are really simple setup manuals and don't have info about advanced setups.
Reddit had little info. I found this post from several months ago that went unanswered. Seems like your same issue.
Is anyone familiar with setting up a Pihole on their fast 5290?
I have set up a Pihole on my local network and when I try to configure my router DNS settings it cannot establish a connection. I've tried configuring my DHCP server a few different ways as well, but I don't think I'm doing it right. I've tried manual DNS on 8.8.8.8 and it works, but when I change it to my Pihole it doesn't work. I can manually configure DNS on individual devices with the Pihole and it works, but I'm wanting to use it for a couple of fire sticks which do not have manual DNS settings. Would anyone be able to give me some direction with this specific router? If I can't figure it out, I plan to get my own router, but I would like to get this up until then.
If all else fails your can turn off your DHCP on your router and use the Pihole as the DHCP Server as well as DNS. This guide has some info on that.
Hopefully you can find the option in the router but its not something you can change.