What retirement plans do I have available to me while on a research fellowship?

I am a researcher in the US on something called the National Science Foundation Graduate Research Fellowship Program. So far, there has been some funky rules with taxes since the fellowship and its stipend do not count like a regular employment salary like a Teaching Assistant or a Research Assistant would be since it is not an “employment” and rather a “stipend”. For that, I’ve had to calculate and prepay federal and state taxes throughout the year. I don’t have a W2 form and instead get a T-1098.

Since I don’t have access to an employee 401k, what other retirement savings can I invest in? I have a Roth IRA but again, this dosnet count as income, so I am unable to contribute to it based off my taxes. I have savings that I earned before going to graduate school and I would like to properly invest that instead of letting it sit stagnant in a HYSA since I think it can do more with the time.