@cjferoussis
I am not from TP-Link, so I would not know, but even TP-Link Support that checks this forum is not privy to that info.
I can see you have M4(R) hardware V2 and S4 hardware V2. You may be in luck, check this forum topic, under the table of the first post: Smart DHCP on/off option under Access Point mode
If you get beta firmware for M4(R) V2 or S4 V2 with that feature, you will need to promote updated M4/S4 to Main Deco: Changing the Main Unit for your Mesh Network
Another option would be to get Deco model that supports turning off Smart DHCP and add it to Deco mesh as new Main Deco. How to Set up a new Deco to be the Main Deco
I would recommend M5, they all regardless of hardware version have that feature.
You know that in Access Point mode you don't need to pass all traffic through Main Deco, right? You can just add new unit as Main Deco, connect it anywhere to your LAN and you are done. Of course, ideally, you should connect it to free Ethernet port of your router/gateway.
These options will not fix the underlying root cause. Deco Smart DHCP will be enabled if primary DHCP server in your LAN goes down. Usually, as soon as primary DHCP server is back, Smart DHCP will be turned off in Deco mesh.
The question would be, why would you lose primary DHCP server, which is not good. Another question would be, why does not it restart DHCP service after it is back. In some DHCP servers there is a configuration option "do not start DHCP service if another DHCP server is running." If yours has it, turn it off and problems with Smart DHCP should go away.