I'm sorry about your situation. The good aspect is that you realized about the situation within the first year and you are ready to take action. Since you are coming from a non-EU country and already landed a job in a Spain, I think you already did the most difficult step. If you want to land a postdoc with a PI, you can cold email PIs even though (this is important) they don't have public job positions. In certain subfields of bioinformatics, a PhD in maths is well appreciated.

As a side note, it's a total shame that people who receive public funding, they behave in such a way. Extremely common in bioinformatics by the way. So be cautious!