Plastics are generally not supposed to be glued except for a few like styrene and acrylic (plexiglass). Many are formulated to resist chemical attack which is what most glues try to do.. to melt and join the surface in a controlled way.
My best sucesses with plastic repair is by 'welding'. I use a soldering iron. It helps if the iron has heat control and you keep it just hot enough to melt plastic. Overheating some plastics makes them very brittle and weak. If you see lots of smoke or black soot, its too hot.
If you must add extra plastic to fill a crack, find a piece of the same type. Cut some off the part itself or find some similar plastic from another part.
The deeper the melt goes the stronger the weld will be. The part will get soft and it's sometimes hard to keep things in line. Practice on some scrap first.
The cheapo-tool stores like Harbor Freight sell plastic welders for about $40 or so. It uses a stream of hot air and a rod of plastic to joint parts, very much like welding metals. I have one and they work but I haven't done a lot with it.
If appearance is important use the same colors and sand and polish the joints.