There are a few options:
Install a "bulkhead fitting" over the leak. This is a mechanical solution and can be good if you need another vent or drain anyway. You have to have access and the area usually has to be either flat or only slightly bowed.

There are bladders that can be inserted into a tank and used independently of the ability of the existing tank to hold water. There is also a resin impregnated version that you insert, blow up with compressed air and allow to harden. These get used a bit in boats where an old tank fails and removing it means disassembling the whole thing. They aren't cheap but they work pretty well.

If you can get to the spot easily you may be able to weld the plastic. Plastic welding uses a hot air gun and a rod made of the same material as the tank. If you're really handy you can do it yourself. If not, or you don't want to risk it, you can hire a guy. Any shop that deals with custom plastic assemblies will know someone. Down side is that welding can be iffy on some plastics, less so if you are experienced, and in some cases older plastic has lost so much plastisizer and cross-linking that welding is impossible because the strong weld material breaks off the older/weaker plastic.

Glues, paints and coatings generally don't stick well to common plastics used to make tanks. Generally mechanical patches work better. I've seen a patch on sewage tank on an RV last for decades. They drilled out the small hole, installed a stainless steel bolt with a stainless fender washer coated with Boatseal on both sides of the tank material. They dropped it in place through a handhold in the top and just snugged it up. Coming back to tighten up the nut after the sealant had hardened. The bolt and washers hold the pliable sealant in place so it doesn't have to stick. In essence it is simply acting as a form-fit gasket.

Any/all of those have/might work but all of those have failed. A lot of times they fail because people try to rush things and are not meticulous about doing it right. Other times such repairs are a lost cause no mater what you do. Often it is best to replace the plastic tank. They aren't terribly expensive. If the people who installed it were good they arranged for there to be some way to get the beast out without disassembling the world to get to it.