No Xfinity Gateway in package

I recently moved and signed up for a plan with Xfinity. I requested a modem. Today I received my self-setup package, but there's no modem in it--there's a spot where it should be set, but in its place was a UPS return shipping label. Everything else I expected is there, even including a plug for the missing modem. The package hadn't been tampered with--this was a packaging error on Comcast's side.

In the Devices section of my account I see a modem listed (model TG4482A, with a specific serial number).

Needless to say this is a lousy way to start (or not start) my new plan. I'm looking for help finding this unshipped hardware (do let me know if there's a better place to raise this specific issue).