The problem is that the yeast has run out of steam. Did you happen to note the size of the yeast packet it came with? Needs 11/12g for a 23L brew, some kits skimp on the yeast and only provide a 6g packet, and then the brews get stuck at 1020. It's a very common problem and lots of threads on here about it. Unfortunately you needed more yeast at the start and not the end, so adding more yeast now won't make a difference as most of the fermentables are used up. You could try adding 200-300g of sugar dissolved in a bit of warm water, and give it a gentle stir to rouse the yeast: sometimes it works, but there isn't a foolproof way to get a stuck brew re-started.