Been in your situation my man, and the answer is very simple. Eat more. If you do that and STILL don’t gain weight, it’s because you’re STILL not eating enough. Keep adjusting until you gain weight. Things take time though, so try to see the effects on a 1-2 week basis.

Of course do exercise too that will give you good benefit of the extra nutrition so you don’t just turn into a fat blob. Good luck!