The best way to repair plastic is welding. Imagine sort of a solder iron mixing the plastic filler with the existing pipe. Be gentle and patient!
Knowing the type of plastic for proper bonding with the filler rod is key, it looks like HDPE. Do a trial in a non-critical area first.
Look up plastic welding on you tube.
The second best way would be a two part Poly-weld adhesive. Again do a trial first to ensure a good bond.
If you decide to go the second way, I'd fill that rib first, with a product called Great-stuff expanding foam.
It will ooze out, don't worry. Wait for it to dry, then use a wire brush to clean the area down to the pipe.
Apply the adhesive patch over the boo-boo.