It appears the toilet flange is glued to the drain pipe. Will the backerboard go under the flange or flush with the flange. We are installing ceramic tile on top of the board.

From 1/2" below to flush with the flange is fine.

Hello Popsters1,

Open the link below and will tell you exactly what to do, good luck.

John

https://www.askmehelpdesk.com/plumbing/resetting-toilet-after-tiling-338406.html

I would put another wax ring on top of the other to make sure it seals if you are in doubt