Xfinity is one of the popular choices of routers with high speed internet. One of the major issues is unicast maintenance ranging- no response received- T3 time-out. There is a variation in frequency because of it. At times it is resolved itself and at times you can continue facing the error and there is no internet connectivity. Make sure that the cable connected is not damaged. The connector can also be faulty. If there is any physical damage to the connector, you should replace it. If you still face the error, call the company and replace the modem.