Connect a computer via Ethernet to the modem bypassing the router. Check to see if it works and also see what speeds you get. You should get close to the speeds you are paying for. This checks the modem and your ISP. If you have issues, check the modem logs if any and you may need to contact Comcast. Once that is working, then reattach the router. Power down the modem and router for 5 minutes (signals the ISP to check the modem's firmware). Power up the modem and wait for it to stabilize and then power up the router. If you still have an issue, reset the router to factory specs and reenter your settings.