What can I do if I was charged with robbery yet they didn’t file they case but then issued a warrant 5 months later?

Is the case pending in court? Did it get dismissed? If it was dismissed then you do not have to do anything. If it is pending in court you have to get a lawyer and fight the matter.

The DA can file charges any time within the statute of limitations. It could be that they were still investigating the case and decided to file months later. Your lawyer can look things over to make sure there are no procedural issues, but the main focus is that you're now facing robbery charges. You could be looking at state prison time and a strike if convicted. Get a lawyer. Look for the best criminal defense attorney you can find that routinely practices in the court where your case will be heard. In the mean time, do NOT make any statements to anyone about this situation.

You need to retain counsel and surrender on the warrant.

It can't be a DA reject. A warrant was issued for your arrest because charges were filed against you and you failed to show up in court on the date you were supposed to.

Nothing, the statute of limitations had not run.

You can hire an aggressive criminal defense attorney in your area. That is the very best thing to do.