Well, these are indeed solid workarounds. The first one is kinda for me... BUT - 

If one connects pihole device to the "outter" router, and the wi-fi clients are connecting to Deco, you will not see which device visits which web address. It will all be showing the Deco IP. Sometimes it's good for diagnostic stuff to know which device is connecting to fishy websites etc.

The second workaround, if I understand it correctly, requires me to buy additional device for it.

And finally, we shouldn't have to use workarounds for such basic functionality. :(

It doesn't have to be two separate devices, it can just be two different interface cards on the same server.

On my setup that's how I have it. My DNS server is running on a Dell thin client which comes with a 1Gbps NIC on the motherboard and an empty M.2 E-Key slot normally intended for a wifi card. Instead of a wifi card I instead bought an Intel 2.5 Gbps NIC to slot into that slot. the 2.5 Gbps is plugged into Deco's main lan while the 1 Gbps NIC is plugged into the outer router's lan. The two NICs have two different IP in two different IP ranges. On the Deco I have the two IPs setup as primary and secondary DNS. So which ever DNS server IP users go for the end up at the same machine. Just the secondary DNS is sitting outside of Deco's lan on its wan side so guest network can reach it fine.

One machine with two NICs would be a lot easier to administer than two separate machines.

  @TP-Link Shameful that this has been so often requested for so long and still has not been implemented in some shape or form.

Hi @Temstar

Thanks for your workaround.

I have an alternate, software only workaround (using single ethernet connection) for scenario 2 (where DNS server is also running other internal services)..

https://www.reddit.com/r/TpLink/comments/1tt5o5q/guide_guest_network_local_dns_server/

Seeing as it looks like tp-link are never going to fix this :(

All the best.