DAX Dense Rank Calculated Column based on Prior Sales Amount Values

From the follow data set, I am trying to use DAX to derive Column H, which is the ActivationNumber. When a Product is first activated, there is no prior Sales Amount, and the ActivationNumber = 1. If the Company has a gap in its purchase history for that Product, and then purchases it again later, the Product is "reactivated," so the ActivationNumber iterates and becomes 2.

I've also included "PriorAmount" and "ActivationDate" as possible helper columns. I'm able to derive PriorAmount using LOOKUPVALUE(), but unable to derive ActivationDate with DAX.

Any help would be greatly appreciated on the DAX calculation for Column H, "ActivationNumber." Thanks!