Yes, I want this feature. The current situation is frustrating. 

  @drewscoop They will not fix/change this. That much is clear! 

I was forced to get rid of their router and go with ASUS because the router has a tough time supporting more than 40 devices even though it can handle more than enough bandwidth. It would shut its wifi down randomly all the time once you hit a certain limit on connected devices.

THe ASUS router I am over 60 devices and it works just fine and supports all the features I need.

I failed to mention that TP-Link support is horrible!

Good news everyone. Managed to setup custom dns on my home guest network!

Bad news. Sold all my TPLink equipment and went with asus, oh well

@TP-Link I can't even imagine how the devs of a networking device did not think about enabling custom DNS settings if they allow setting up a guest network.
I am a chronicler for a youtube and podcast channel where I was offered the option to test this equipment. I was impressed by this product at first sight and recommended it to multiple contacts and in the podcast and youtube episode.

If you don't give a feedback and timeframe for this feature, I will change my public opinion on this device and will recommend NOT to buy your products anymore.
I will also invite other users to share their bad experience about this.

I really don't like proceeding like this but your lack of reaction for such a long time for this feature that is a MUST HAVE, is a shame.

  @TP-Link the DHCP thing was annoying with the guest network but what is more annoying is the device limit on it. When you get over 40 devices (the exact number I am not 100% sure) the WiFi will start to randomly drop out. This happens when you upgrade to their latest fancy Firmware. If you are on the old interface (before they changed the look) then this issue does not exist.

Their support was not helpful and reverting back to the older version of the BIOS (they gave me instructions) appeared to be very difficult.

As I mentioned in an earlier post, I got fed up with these two major issues so I ended up getting an ASUS router and all of my issues went away. I am up to 70 devices and can control the DNS for guest network however I wish.

It is very easy to get this many devices now a days with smart homes.

TP-Link may be cheap and have nice specs on paper but from what research I did these problems are not isolated and it is notorious of TP-Link to be this way. They are basically a budget router.

+1 vote for this feature. In fact, I have almost the same exact situation @Lothos mentioned as well. I have 50+ devices and my network is incredibly unstable. I also am running two Piholes wired to my primary network which means my Guest network won't work. @Lothos, I sent you a DM. Please let me know there or here what Asus router(s) you went with. Very interested. Trying to determine if I go with ZenWifi or one or more routers and go the Aimesh route. I'm within my return window with Amazon and plan to send these back due to the issues you and others have mentioned that continue to go unresolved. Very disappointed in how unstable and the lack of core functionality that exists.