My experience is that HRMs don't give good readings with A-Fib. It gives you some idea of the general level of effort, but you will see lots of numbers going up and going down as the unit tries to make sense of your heartbeat.

I tried several models, including some rather expensive Polars, and nothing worked.

Fortunately for me, I've been successfully converted to sinus rhythym, so I don't have this issue any longer. (and like you my cardiologist encouraged exercise, even with a-fib)