ISP Issues

It's possible that your Windows and Mac OS are attempting to acquire an IP address from the ISP at the same time, and if you're only permitted one IP address, it could be blocking you. Another possibility (and I haven't worked with Bootcamp, so I don't know how it works or if this could indeed be a problem with Bootcamp) is that if you've configured Bootcamp incorrectly as far as how it obtains an IP address, you could be completely killing your internet. I know when I was setting up a virtualisation application on a Windows laptop I use for school, attempts to configure a static IP address and set the DHCP server address using one process were met by problems that killed my network access for ALL devices. Going in to configure the settings using another process resolved the problem.

Firstly, check that it is your modem's MAC address being read by the ISP, or the if the modem isn't set up that way, your router's. You can set up the router to clone your PC's MAC if you're so inclined.

Next, your router should be configured as a DHCP server and get its own IP address from your ISP's DHCP server. Check your network settings on your two OS - Windows through BootCamp and the Mac OS - to confirm both are getting their IP addresses via DHCP, with the router being the DHCP server.

If your Router's IP address on your network (LAN) is 192.168.1.254, both OS should have an IP address of 192.168.1.xxx (depending on the DHCP range you set up on your router). The router should also be the only device with an external IP address.

Hope that helps.