Relay DHCP from another Non ASUS Router / Device

[SABOSO]

Is your feature request related to a problem? Please describe.
I am using my ASUS DSL-AX5400 on 3004.388.10_2-gnuton1 to provide WIFI to client devices.
I'm frustrated that i can disable DHCP but cannot find an option to relay DHCP from my Ubiquiti Cloud Gateway Ultra.
Ubiquiti Cloud Gateway Ultra is connected to the WAN port on my ASUS DSL=AX5400.

Describe the solution you'd like
These modems are getting old now, but would be an excellent AP for the next few years. If we could do a DHCP relay from another router (non ASUS) it would be excellent.
Otherwise now i have two different IP Address ranges.
If they were on the same range, where it is relaying the DHCP IP address to WIFI clients, I would be able to view those devices on my Ubiquiti clients list giving me greater control.

Describe alternatives you've considered
DHCP can be disabled, can use manually assigned IP address, but that means at least 2 different IP subnets in a house.
Also painful for WIFI devices to assign IP address manually.

Additional context
Thanks.