Math math and more math.
His series on the Art of Computer Programming has enough relevant mathematics (among other things) to get one busy for a long, long time.
Mathematics per se is too wide a subject (which may be why the word is given in plural in English and French). It is also extremely deep and complex even in its "universally reusable" parts, and, frankly, not much of it finds application in software engineering.
My own situation is this: I've been working as an RF telemetry and instrumentation engineer for 8 years. Time and time again, I've been able to rise above because I put in the time to understand the fundamental math underlying what we're doing, and use that understanding to troubleshoot issues. Surprisingly, most of my colleagues have not put in this level of effort and their career trajectories have suffered as a result.
I would also add this: learn to program. Now, the vast majority of people here already do know how to program, but many people in the aerospace and mechanical engineering fields really don't, or don't use what knowledge they gained in school. I put in the time on the side to learn some data science and programming skills. I was able to use these skills to solve some rather tricky problems, gaining me additional recognition. I have now used this success to switch career tracks to a software engineering position. I start the new position next month, wish me luck!