Wow, I just spent some time browsing these forums and other forums - TP Link has been asked for this 1000's of times!  This is obiouvly-needed functionality and to see the "Future consideration" tag on this thread aggravates me.  They clearly have no plans to fix existing devices.  My Amazon review is 1 star and points to this matter.  Until I see this fixed on existing devices, I'll will be recommending against TP Link from here on out.

  @richpriest yes, I did the same!

After wasting 1 year of my life with TP Link, I sold my device on Ebay, moved on to Ubiquity (if you want less fuss, go with Asus).

TP Link cannot provide support (my BE65 got 1 firmware  upgrade across the whole year) .. and treats their premium products like the cheap product lines.

Dont waste your time, give them to someone else who are not enthusiast or wants to play with their devices.

I'm also thinking about changing to some other mesh devices, since I can't use the guest network without those settings available, and because @TP-Link didn't even care about this since my last post, not to mention the OP and hundreds of other users... 

It's actually a matter of users' networks safety!

  @richpriest Does anyone know of a brand/model that would work for what we need?

  Hi, 1+ from my side for this feature. Same usecase as most of you (pi hole, guest and isolated IOT network on the Deco BE65). In my opinion its relatively easy to implement such a feature. The only alternative i have in mind are multiple subnets with vlan to subnet mapping and  mabye intervlanrouting (FW). I would really appreciate to have the possibility to configure a custom DNS Server for Guest and IOT Network.

  @richpriest this is just what I need, let me just type my local DNS IP in the guest network settings that it will just work.