How to assign a static IP address

I can't find a way to assign a static IP address to a connected device via the Xfinity app. I used to do this by logging onto my XB-7 modem directly via a web browser. But entering http://10.0.0.1 reroutes to myrouter.io which gives me nothing but an error message: The page's address isn't valid.

Really, Xfinity? [Edited: "Language"]?