GL1200 Opal x3 - 1 router , 2 extender

I have 3 of the 1200 Opal routers.
None of which will be Internet enabled. So turn WLAN to LAN. (got that)
1 Main will have 2 wired connections - PC and a controller (100m Ether connection)
Other 2 will have a singled wired connection each (same model controller.)
Want them Wifi to Main so I chose Extender selection within More>Network Mode.(got that)
How can I assign them a static IP?
Assume their DHCP is disabled when in this mode?
(as I dont see a way to turn of DHCP?)

The controllers have assignable static IP’s
But I dont know MAC address and am currently trying to figure that out. As I assumed I would assign those within their respective router.

Am I on the right track?

Any chance of a diagram of what you are trying to achieve, is this a local non internet WiFi configuration all local network?

Closed local network without Internet.
Router 1 - LAN 1 = computer , LAN 2 = Controller 1 (assignable static IP)
Router 2 - LAN 1 = controller 2 (assignable static IP) wireless to MAIN
Router 3 - LAN 1 = controller 3 (assignable static IP) wireless to MAIN
No WAN connections.

From Router 1, you can reserve the Repeater IP addresses.
DHCP is turned off in Repeater mode.
Controllers static ips would also be reserved in Router 1.