Okay, so were you able to get your ACH info from the web site?
Thinking a data point on how the payment method was not accepted by your HOA site might be useful for other people who are looking into this card.
Found the ACH info online - not sure why that was hard for me, but it's there under "Pay Rent".
I don't see any way to get the Mastercard number on the website (which is doable in the app).