About termination

I work in US based company..I was suffering from covid was on probation period ..i request for WFH many times...HR said the decision is in hand of supervisor & supervisor decision can be taken by higher authority only....i was very ill had post covid symptoms had facial swelling & pr bleeding,brainfog,muscles pain...so i join the office again when i get back .they started treating me poorly my team get change...my new TL targets me without no reason.. i was taking post covid treatment that time...he was not approving my leaves..even they were unpaid...& at 3 sep i took half day & provide him medical certificate...request him &told him ...sir as I'm suffering I had to take the leaves..he said ok...& even though that i received termination letter.. that I'm absent from 2 sep without supervisor permission &he didn't inform to HR...as they were mentally harassing me...i quit the job.they didn't gave me relieving letter which i want..as per bond...they are saying we can issue no due letter..even though it was their mistake & i request to pay reimbursement & give me relieving..& now I'm searching for another job..but they are saying so many bad things about me to my new company Hr...so what should i do