When you plug in the ethernet cable, try going to command prompt then using ipconfig /release. After that completes, use ipconfig /renew. This gives your computer a new IP so the modem / router can potentially work with the new IP. If that doesn't work, try unplugging the modem / router for 15 seconds, then plugging it back in. This will renew the modem's IP so your computer can potentially work with it (I toss "potentially" around quite a bit cause I can't guarantee it'll work).