@tonyntas thats what I'm doing in some sort. Got 2 ranges 192.168.0.0/24 thats the one from my ISPs router created a vlan on my switch for that. Added the vlan to proxmox and added an container with a second dns only for vlan one. The deco has his wan ip in that range so I set the second dns ip as the dns in de 1e network aka 192.168.0.20 while the network range in my deco network is 192.168.178.0/24

Hope this helps.

@TP-Link 

Any update on this ?
The recommended solution isn't a solution or workaround at all

Having primary as pihole and secondary as cloudflare defeats the whole purpose of pihole in the network

ALL NETWORK TRAFFIC SHOULD GO THROUGH PI-HOLE to filter traffic

Thats the main reason and having different secondary makes the device choose the secondary more often

Custom DNS option is the only way to do it.

 @TP-Link 

it's been few months since we had any updates  and peopl are wondering what is the state of this?

there have been many suggestions on how to fix this..

Worst case scenario... We cannot use DNS from the main network.. fine, give us a another public DNS option.. something that will allow internet  in guest network to work... even if it means we cannot block ads in Guest network or use pi-hole or any "main network device"

Here some inspiration on how Asus has done it

+1 here

  @TP-Link this need to be added asap! Option to  manually set DNS from local server for guest network. It's really annoying that iot and guest network not working unless I use isp or google etc dns 

  @TP-Link Please allow us to have separate DNS settings for the guest network. Like it was mentioned here, it is not a feature request, it is a BUG! This should be essential, every other router I worked with had this option.

Deco seems to me as a solution for dummies or kids, so much reduced functionality! I'm very frustrated each time I want to do some improvements in my home network, Deco just stops me. Only staying with Deco because I've spent a lot of money for 5 units. Will wait to next update, then going to throw all units to trash.

  @matoB I don't understand why people keep posting on this thread. They haven't fixed the issue in over five years, and this problem was known to them even before COVID. Due to their poor support, I ended up getting rid of all my TP-Link hardware, which I found to be mediocre at best. Despite the impressive stats, it never fully delivers. For example, my router struggled to handle more than 50 devices, and anything above that would cause random disconnects. This issue started after the latest firmware update, which I reported, but their response was indifferent.
 

I switched to ASUS routers, and now I have up to 80 devices connected with no issues. It handles everything I need, including DNS for the guest network.
 

TP-Link products are cheap for a reason. Unfortunately, I learned this the hard way, just like many of you.