Ashkenazi practice is to count the five-day minimum wait from the onset of niddah status or of halachic prohibition. This applies even if the couple could not have had relations for some time beforehand (e.g., they were in different cities). So your husband’s quarantine should not affect this halacha.

In cases such as yours however, where you have already endured a prolonged separation and where shortening the five-day wait would make a material difference, given the unique stresses surrounding immersion, you may perform a hefsek taharah as early as you can get one and begin counting the clean days that evening.

May we all merit good health and good news.