On the back of my boat, there is a bait board cover made of plastic. I think similar to hard chopping board plastic but is somewhat smoother.
As swimmers tend to stand on it to climb aboard, I need to make it non slip. In the past 12 months, I have tried 2 methods without success.
I have lightly sanded it, cleaned in spirits, applied 2 part UV proofed epoxy then sprikled salt on it. This method had been used successfully with boards before. But the epoxy wouldn't stick and I had to scrape the lot off.
Used outdoor rated non-slip tape from Bunnings. This lasted a bit longer but coloured and peeled off.

Ano other good suggestion?

Waterproof non slip stuff like sandpaper, applied with good double sided tape - 3M VHB is tops, you could stick yourself to the ceiling with it.

Sounds like it's made with polyethylene, which you can't really 'glue' anything to satisfactorily. You could really rough it up to get rid of the slip, or heat it up and sprinkling sand or grit onto it (if it's not a thermoplastic you might regret this), or stick something to it with some really good aggressive adhesive tape (as suggested by Mark). Sorry to say that the fact you have sanded the previously smooth surface probably now means the adhesive tape will not stick to it as well.

That is the stuff tools for working with glues are made of so they can be cleaned and reused.
Maybe get some Sika 206 primer after roughing it up, then some white 227 Sikaflex and spread it around. It should be soft enough to offer some grip without grinding skin off.

Put some EVA on it, like a SUP deck pad.

You could also paint it with epoxy clear coat or something like that and sprinkle sugar on it. when it dries wash the sugar off (it dissolves) and you're left with a rough surface. it worked on the skatey