Support? How? Who?

I purchased a subscription a while back, had uninstalled it, and now reinstalled it from the Windows Store. How do I tell it that I have a subscription

You will need to contact the MS Store for assistance. It is possible you are logging into Windows with a different Microsoft account than your MS Store one so it doesn’t recognise you, or maybe you have stopped paying the monthly subscription through the MS Store for the eM Client Pro features, or there is some other issue with the MS Store or your account. They will be able to tell you more.