@Ardeen 

I'll just try similar connection, I have to check how it is working

@Ardeen 

For Deco in router mode and use all resources:
 

TL480+

Wan 1/2: Dynamic (Get from ISP Router)

Lan:

IP:10.100.100.1

255.255.255.0

DHCP ON

Static DHCP Server entry: Main Deco 10.100.100.100 (Wan IP of main Deco)

DMZ Host: 10.100.100.100

UPNP On

Deco Main on LAN of TL and all your network devices works with 192.168.1.X IP's provide by Main Deco . No issues.

If you want Deco in AP mode:

TL480+

Wan 1/2: Dynamic (Get from ISP Router)

Lan:

IP:10.100.100.1

255.255.255.0

DHCP ON

Static DHCP Server entry: Main Deco 10.100.100.100

DMZ Host: 10.100.100.100

UPNP On

 all your network devices works with 10.100.100.X IP's . With no issues too.
You can disable Wifi on ISP Router, don't turn off DHCPd

@Ardeen I am doing almost the same thing. but I am plugging my Deco's in AP mode directly to the Dual Wan router's switch. I am using a LRT224, with one deco connected it works. If I connect the second deco to the switch and add it to the app the app can't talk to the network any more.

If connect the decos to the ISP's router's switch without the LRT224 it works. 

If I connect the main deco to the LRT224 and all my other deco's to the 2nd port on the main deco it works. 

The problem is I need my other wired device not to be connect to the decos, it runs to slow. If I connect my eithernet for the different rooms I can segment what is connect to the main deco, because I only ran one cable per room.

My issue has to be some kind of port forwarding issue