If your home is wired with coax you will be better off swapping out the xi5 boxes for xid boxes instead. They have be hooked to comcast via coax. Also, note that you will need a moca filter somewhere before any splitters (such as by having a moca filter ground block). You may already have a moca filter. Check your house box outside. Google moca filter to learn what it is. It is necessary for your X1 system to work right.
By getting the Xid boxes instead you will not have to rent a modem from Comcast.