I just had this problem and talked to someone from Comcast. He suggested adjusting the security settings on the router. If your security is any of the "AES" settings, the Fire Stick won't connect. You have to open your browser, go to 10.0.0.1 to access your router. The login is: admin Password is: password.
Go to Connections: wifi. Edit security. You can see the list of security setting types. You just have to change it from AES to the TKIP encryption type. That should solve the problem.
Note, this will change your network settings so you will also have to change the security settings on your other wireless devices for the new encryption type.
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