How can I quickly learn advanced math concepts for my PhD in engineering?
The truth is: There is no magic bullet. You didn't learn engineering in two months, and you won't learn math in two months.
If there are specific mathematical concepts you need for your work, you can of course select books from that area. If self-study is not fast enough, you can also consider paying a tutor to walk you through material. But short of that, mathematics is like any other field and language: It requires years or studying and practice to become fluent and productive in.