Mac can't connect to internet via ethernet when wifi enabled

I am having an issue with my Mac connecting to the internet. I have recently upgraded my internet service and installed a new modem with wireless built-in from my ISP. I have ethernet wired to my Mac, a PC, and Apple TV box. Several other devices are connected via wireless to the modem/router and function properly. I also have a wireless enabled printer connected to the PC and I use wireless to access it from the Mac. The issue I am having is when I enable wireless to connect to the printer, after about a minute I lose internet access from the Mac even thought it is still connected via ethernet. As soon as I disable wireless on the Mac the internet access is restored. Any ideas why this is happening?