@vmonkey 

If you don't mind, a question about your network setup. I, too, have Synology NAS. I just checked its standard DHCP server: it is very basic, if not primitive. Very few configuration options. Can't figure what is so special about it.

If you run Deco mesh in AP mode, you must have router in front of Deco. Why would not you use router's DHCP service? 

As for the issue you raised with TP-Link Support for M4 V2, see post #1 from the following: https://community.tp-link.com/en/home/forum/topic/519144

  @Alexandre. Thanks, I will send a private message as suggested there.

You are right that Synology DHCP is not advanced at all. Nevertheless, it is sufficient for my needs and the router one is not. (Vodafone custom firmware for a cable modem. For example, there is no way to set own DNS). You may ask why not use Deco in router mode then? I tried that but the response seemed to be considerably slower.

  @vmonkey 

You make a good point with cable modem. I can relate: my Canadian ISP recently replaced my cable modem with new model and I lost the ability to configure DNS servers. I was using AdGuard on my Synology NAS, now I can't. 

What must be happening is DHCP server on Synology also having some sort of Smart DHCP feature, which is "don't start DHCP service if another DHCP server is active on the network." Just like with TP-Link Deco, it is not documented. Just like with TP-Link Deco lineup before, it can't be disabled. Both Synology and Deco Smart DHCP collide in your case.

To be fair, one should also escalate that to software developers of Synology DHCP server, but I suspect their response will be even less enthusiastic than from TP-Link.

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Hint: if customized firmware for M4 V2 with SmartDHCP disabled is older than your current firmware version, there is a chance you could still push that customized firmware to your M4s by using Deco firmware recovery process. An extra work, but consider it to be part of the lesson learned not to upgrade non-GA firmware with GA version lacking feature you need.

I have the same setup on 3 floors: VDSL router on the top floor, connected to a switch, where everything else connects. Decos are on the two other floors (ethernet), using AP mode. Every time there is a network disruption, I have to manually reboot them. DHCP is on a Synology NAS. Router, switch and NAS are all on UPS, so the Synology DHCP practically never goes offline. I like to use reserved IPs so I know which devices are connecting and have no need for a more "advanced" DHCP. I do need to disable any "rogue" DHCP which I cannot configure. I also prefer not to have the double NAT translation forced by router mode.