How do I turn off DHCP in the Asus routers

I have 3 Asus rt-ac68u routers in AiMesh Node. These are connected to an Arris-2192 Gigabit Modem. I am getting some Double NAT errors. I assume it's because both the modem and routers want to assign ip numbers to the devices down stream. How do I turn off DHCP in the Asus routers without messing up the AiMesh (which was not all that simple to get and running) ?