Xi5 wireless box doesn't recognize my Comcast router.

Is it possible it’s too far away from the modem? Move it temporarily where have successfully activated the first one

We did just that and moved it 5 feet from the router......no luck. We've even exchanged the Xi5 twice so far thinking it was defective, with the same results for all units.

Can I try hooking Xi5 to the router via ethernet just to get it up and running?

Yes you should be able to but once you disconnect the ethernet it will look for the wireless network
Did you try rebooting both the modem and the xi5?

Ugh! Rebooting all equipment still didn't work.