when not allowing remote requests is turned off the Debit/ credit card machines hooked up to the router would not allow a transaction to complete. Would you have another suggestion?

I just received this email below from my ISP....I am new to router OS and Im looking for some help on fixing this.... "You are receiving this email as it appears your system is running an open DNS resolver - this is usually due to an unnecessary service running on your wired or wireless router....