Using old Tomato router as Pihole DNS for main router

I want to use my old Linksys E900 router as adblocking DNS server (a no-frills Pihole) for my entire LAN. The E900 will run behind an EdgeRouter X (ER-X) and the E900's IP will be propagated as primary DNS via the ER-X DHCP for all the LAN devices.

I have fiddled with it extensively, but have had no success. The main router is an Ubiquiti Edgerouter X.

I need the step-by-step...

Using old Tomato router as Pihole DNS for main router