I do not know anything specifically about the "blocker concentration" of Bisoprolol , but if it is anything close to that of Metoprolol (a common generic beta blocker used in the USA) I'd say 5 mg is very low dosage. I take 50 mg of Metoprolol and get a considerable reduction in my AFib driven heart rate (down from a rest rate of about 130 to something in the 80s). This makes me suggest you have a discussion on increasing your beta blocker.
Do you experience any side effects from the Bisoprolol ? That is one consideration on setting a dose level, that is if one is having problems with side effects from a drug, it seems unlikely that one would want to increase the dose size.