I got a new Comcast/Xfinity cable modem recently and it was reset and became a router instead of a bridge to my router which caused my Slingbox to disconnect from the internet... Had to call XFINITY support to reset my modem as a bridge and after 2 weeks of not working outside my home, it now works on my cell phone using the Cell Netowrk!!! Hurray... Check to see if your new Comcast Modem is a Router or a Bridge...
We are sorry to hear that you haven't been able to access your Slingbox. When you change the modem or the router you will have to reconfigure the Slingbox. What you will want to do is reset the Slingbox and then go back through the setup again. You will go to setup.slingbox.com and then the setup process will begin.
Hope this helps,
Sling Moderation Team