Getting a job offer while the employer refuses resignation. What to do?

I have been working in a company in Ajman for the last 4 years. My labor contact is limited. It will finish on 19th January 2022.

Now I got a job offer letter from a Dubai-based company with a 1 month notice period. I sent a resignation letter yesterday for the 23rd December, but they refused and asked me to stay for 2 months more.

I have to join the new company on 25th December. What to do?