I resigned during probation and the employer holds passport and cancellation

I worked my 6 months probation and after a few months got an offer. I gave my resignation letter which my boss signed as "not accepted".

I asked and they told me it was illegal. I stopped working on my resignation date, but they sent intimidating messages that they are going to put absconding on my visa.

They won't release my passport, they won't pay me or cancel my visa, and I have to pay visa costs.

What should I do, because they told me they have bigger lawyers I can't win them.

How can I get helped?