My local Xfinity Store Doesn't have WiFi Extenders

I signed up for a new Xfinity plan with Xfi Complete which is supposed to include an Xfinity Gateway and a WiFi extender. Yesterday, when I went to the local Xfinity store to pick up my equipment, they said they didn’t have the WiFi extender. How can I get one?

I’m paying for 1100 Mbps. The computer by the Gateway gets 980 Mbps, but a new computer in one of the bedrooms is only getting 50 Mbps. I feel like I should get a rebate until the WiFi extender arrives because I’m gettting much less speed than I am paying for.