Is it better to focus on relearning mathematics fundamentals or learn material as needed to understand research papers?
It will not be very easy to learn a broad subject in mathematics by reading research papers.  Books which cover the subject starting from the basics will be more cohesive (for instance, regarding notation and terminology) and will leave you with more approaches to solve problems. If your major requires a lot of mathematics, hopefully you can identify a specific area (like analysis, geometry, probability, etc.) that you need, and find a good text that synthesizes a lot of the research.