Employer not agreeing to give relieving letter

I have been working with a multinational company X for the past 1.5 years. The company has a 12 month review cycle as mentioned in the employment agreement. I joined in February 2022 and was expecting my appraisal in February 2023. But i got to know that the company follows a May to May appraisal cycle. The company delayed the cycle this year. The appraisal letter is dated 1st May but the appraisal letter was actually handed and intimated on 21st June 2023. The appraisal was also not up to my expectations. I was not promoted and the salary increment was also very poor. The company was also forcing me to come to office thrice a week. Since I had joined the company when it was working remotely, I couldn't afford to travel to the office thrice a week in the current pay. Looking at all this, i decided to resign effective immediately since if i had got to know about my appraisal and increment on 1st May itself, I would have already completed my notice period of 2 months by now. The delay on the employer's end made me choose this decision. Now my employer is not agreeing to give me the relieving letter and the experience letter because i am not serving the 2 month notice period as mentioned in the employment agreement. I am not looking for another job and will be pursuing higher education for which i might need the experience letter. What can i do?