Is the R4500 compatible with the CM1100

> [...] The xfinity tech who installed the modem could not get my R4500
> router to work. [...]

   Sadly, few of us know what anyone tried, or exactly what happened
when it was tried.

   Did anyone try connecting a computer directly to the modem?

   It may seem like a radical idea, but you might consider visiting
http://netgear.com/support , where you could put in your modem and/or
router model number, and look for Documentation.  Get the User Manual
and/or Installation Guide.  There should be some basic installation
instructions there.

> [...] Is the R4500 compatible with the CM1100?

   I don't see why not.  Practically any router is compatible with
practically any cable modem.  "Optimal for" is different from
"compatible with", but I'd save that worry for later.