So it has to be at the bend that is going down the drain pipe. There is no evidence of leakage there though. The down pipe is saturated, son it has to be going down there or below there..
Well, I'm no plumber myself but wouldn't trust any type of glue such as that. How about cutting out the bad spot and adding a Fernco flexible coupling?
ask the guy at your local plumbing supply for a plastic x plastic band. get two.
measure between your cut. the subtract 1/4" from your tight to tight measurement
slide the band over the pipe, put the rubber on . fold the rubber back on both ends and put the pipe in place
fold the rubber down, slide the band over the rubber. tighten the band by going from one nut back and forth
DO NOT tighten one band at a time, tighten all of them together
I have never seen that piping before. I google searched it, and it seems that it is or has been made in a thin wall version with the same ID as Sch 40 pipe, just a smaller OD. I am wondering if that would even be code compliant piping?