Ryan got a new modem because his ISP is now giving him faster download speeds. But when he plugs in his router, it slows down to a crawl. Leo says that since the router is new, it should be fine. Try a different ethernet cable. If the cable is old, it may not handle the bandwidth. Then, try another computer and see if you can replicate the issue. If you can't, then that will point to something on your computer. It may be the ethernet port is too old. Also, update the firmware of your router. Look in the settings and make sure your quality of service isn't splitting it up to multiple devices. Turn off QoS.
Check your WiFi. If it's slow, then it points to the router so RESET it. If it continues, then return the router. It's probably defective.