@Fasanen 

There are multiple conditions that could trigger "Smart DHCP" in Deco running in Access Point mode, but in your case I'll guess that happens when TP-Links Archer MR600 loses Internet connection or it becomes really, really slow. Which is not that surprising with cellular Internet connectivity.

After TP-Links Archer MR600 gets cellular signal back and Internet connectivity restores, it should take less than 5 minutes for your office network to recover. 

Not very much can be done about this "Smart DHCP" feature. TP-Link promises fix (will allow to turn it off) in firmware 1.6, but ETA is unclear. You will need firmware 1.6 for both M5 and M9Plus, disabling "Smart DHCP" may not work if only one of these two Deco models gets 1.6.

When you get fiber, that issue should dissapear. It does not present itself with fast and reliable Internet connection.

If you can't wait till any one of above two will happen, your only other option is to switch to different WiFi mesh brand.

@Alexandre. 

Oh, why do the Deco's trigger their "Smart DHCP" if internet connection goes down? Why not check if the DHCP goes down, thats what its there for imo.

Its annoying enough that I need this "cloud" wannabee even to config them at all, not to mention mobile config-interface instead of full web-based one.

Manage to solve it as temp fix by going static IP on both primary Deco and my modem/router, connect a single AP via ethernet using static IP and connect the servers that are a must to get to remotly via wifi to that single AP, more or less creating another network just for that with all static IP's.

Tomorrow Im going get some REAL wifi that works as I tell them to do, TP-Link will not be bought in anyway, shame as I have 12 of these Deco's and a router/modem that are more less e-waste now.

Thanx for the Reply

@Fasanen 

I wonder if you decided to wait a bit longer before decommissioning Deco, because if you did - there is a solution now. TP-Link released firmware 1.6 for Deco M5, see its description here: [New Firmware] Deco M5 1.6.0

It is for M5, but if you can make M5 your Main Deco, at least until firmware 1.6 is available for M9Plus, you should be able to turn Smart DHCP off. You only need to upgrade single M5 with firmware 1.6 manually, and promote that M5 to Main Deco.

In case you already moved from Deco, would you mind to share which other WiFi mesh brand you've chosen and how in that mesh one can choose signal source? Thanks.

@Alexandre. 

We did ditch TP-Link deco system, went for Ubiquiti Amplifi, works like a charm.

Wasnt just the "Smart-DCHP" bug that made up my mind, the total lack on config wia web and only phone more or less was one factor, that I have to be online and go via TP-Link "cloud" to even reach my AP's was more important, I need and want full control over my hardware component that I do own, I want me to have full access to them and not TP-Link harvesting our data how and when I do set them up to help them with R&D for free.

We manage to return 9 Deco M9 to our wholeseller at full refund, the 3 Deco M5 we did just throw away. This was an eyeopener for other TP-Link products aswell, as we used a main router from TP-Link aswell, that did get the boot as I found out it CANNOT be set to bridgemode, something that even the cheapest home router from other brands can be set as. O.O

Regards Fasanen

This is still broken as I have a functional DHCP server in my network and "Smart" DHCP refuses to remain disabled.  It is causing my network to be practically unusable because it keeps stepping over my real DHCP server.  For this product to be useable for those of us who run a real DHCP server (there's a ton of us, clearly) this feature needs to have an option to completely disable.  I don't want it to ever come on unless I turn it on, why is that such an impossible concept for the developers to grasp??  Just let me turn it off, and while you're at it, let me set whatever WiFi channel that I want to use!  I paid $500 for a WiFi system that I expect to be able to use.  It is unusable.  That price point comes with a certain expectation of control, and the power users currently have zero control.  

I have two x90's at the latest firmware in AP mode.  Just want DHCP disabled indefinitely and the ability to select my WiFi channels.  My DHCP server is serving a limited scope of IP's to my network, leaving about half of the range for static IP's.  I suspect perhaps this is why "Smart" DHCP is failing at its job.  Maybe it doesn't play well with limited DHCP scopes in the environment?  Whatever the case, the correct resolution would be to remove the "smart" feature and just leave it off if the user turns it off.  It really is maddening!!