Depends on your modem, from here.
But the way to fix it is just turn on DMZ, some modems call it "Apply device to Public ip".
Most modems settings can be accessed through typing your IP into the URL bar up top on your Browser. To find your IP, just search whatsmyip and then just click the link at the top, it will show you your IP. Copy paste it onto the top, enter. That should lead youto your modem settings. I'm not particularily sure about if this works for all modems. If this doesn't work, google it, google your modem model and how to activate DMZ, or apply a device to a public ip. When you do do this, repair your IP and then you should be done.
This works for me. I do it all the time.