Try clicking the “premium reschedule.” It says there’s a fee but there isn’t. I did that in the October flex because my mic broke and I had to order a new one lastminute. Worked like a charm.
Yes you should call and reschedule, it's highly likely they'll reschedule for free despite being past the deadline. I caught covid before the January exam after the deadline had passed and they have a one time waiver for lastminute date changes.