As far as I know, (although it can vary by location sometimes), Comcast doesn't block server connections for the ports LU uses. Check your modem/router, though. These new ones are notorious for having firewall issues, and even more so at being problematic with adding exceptions. If you've already added an exception in there for it, try restarting the modem/router too.
Be careful after restarting it. Sometimes they will "bug out" or erase the firewall exceptions, and any custom MAC address settings you may have set up. (I've gone through two now, and I'm really not impressed with Comcast's crappy custom firmware for them).