Depends on the plastic..some have to be "ionised". Ie) just pass a flame over surface to be glued.
On boards I often have to reattach/ renew plastic ( Acrylic) footstrap inserts into foam ( EPS) and epoxy/ carbon laminate.
Without a doubt best I,ve found is uv resistant, clear ,low viscosity epoxy resin ( its on ebay) Only problem is 9hr set time.. You must identify the plastic.( some polyethylene) are almost impossible to bond and must be welded, but you can only do that to same material.
I,d try one mentioned.(without a medium (ie carbon or gladd strand) it sets quite flexible. ( Its the working bit of araldite, but that has fillers) If you do use pure epoxy resin mix it exactly and thoroughly.