Going to ask since you appear not to have indicated it. Did you do a hard 30-30-30 reset followed by a power cycle of the Belkin N300 router before connecting it to the Comcast cable modem? Have you updated the Belkin N300 to the latest firmware?
Don't know what version/model nummber of the N300 you are using but you may be able to use 3rd party firmware like DD-WRT or TomatoUSB which may fix the low speed issue.