How to get in a Master program in Applied Maths coming from a Physics Background
Because of your financial situation it sounds like you will need to either gain full time admission to a masters program that includes a scholarship offer

or

Attend part time while you work

Many admissions officers will not be familiar with the Brazilian grading system. In addition to enclosing your transcripts and cover letter, submit proof (if it is not already on your transcript) of your impressive class rank. Use your cover letter to further explain you academic history.

You may need to prove some of what you learned as an autodidact in order to demonstrate your preparedness for the graduate programs you apply to. Either produce research or find a relevant test that you can use to prove your knowledge. If there is not a standardized test relevant to any alleged deficiencies in your knowledge base (judging you based on transcripts) ask the school you are applying to for the right to take a final exam of a capstone bachelor level math course to help prove your knowledge base.

You will have to prove to the schools you are applying to what you learned as as autodidact. If you can do that you may be able to gain admission without the need to take more undergraduate coursework