Having the customer Wi-Fi on the same DSL connection as your business PCs and credit card machines poses a risk to the business as it opens the connection up to unauthorized activity. This will not allow your connection to be PCI compliant and makes your company at risk for violating HIPAA. Having a guest Wi-Fi for customers on your business's main internet connection also opens up the possibility of a customer logging into the modem and changing settings that could negatively impact your business.
Windstream offers a product called "Extra Connect," which offers a separate 10 Mbps Wi-Fi connection and includes modem lease for $10 per month. Call Customer Support to add this guest Wi-Fi option to your account.