Didn't get experience letter

I started my first job in July 2019 with a startup. Since it was mine first job in the corporate, I was not aware that bonds are illegal. The company made us sign a 2 year bond. 3 months later there were salary delays and backlogs. Later when covid started, there were continuous 3-4 month backlogs in salary. Due to this situation(managing expenses in tier 1 city was difficult) I resigned from the company saying that if I don't get the salary I have the freedom to change my source of income (As I was under bond). They agreed and released me a week later. But they didn't completely denied the experience letter and when I requested they told me that this is in our hands and we won't provide you. I couldn't do anything. I joined another company and I am working fine here but to switch further I need experience letter. The previous had not even given 3 and half month salary and Pf of 10 months too. What next steps I should take? Should I ask for experience letter and salary over mail first? Earlier, They had not replied to my mail for clearing salary. Thank you for your time.