Any complex enough problem will take time to solve. You’ll probably focus on it for a long time, and maybe even get so lost in it that you don’t even make progress anymore. You just go in circles.
In that case, take a step back.
When you do, your mind will have to stop focusing, and you’ll be able to take a look around. Notice what is there. It may hold the key for you to solve your problem.
If you have people to talk about the problem with, do it.
Read blog posts about different topics.
Go for a walk.
Work on a different project for a week or two.
Your difficult problem won’t go away, but you’ll see it with an other angle, and maybe will find a way to finally crack it.
Let your brain make the connections.