Why are cursor keys in Excel moving the viewport rather than the active cell?
This sounds like Scroll Lock is somehow activated on your copy of Excel. IF you have a large keyboard with an F14 key, ShiftF14 can turn on and off Scroll Lock. If you don't have such a keyboard, or access to such a keyboard, it gets more complicated.

Microsoft support posted this article about the various methods of deactivating Scroll Lock. They either involve connecting a keyboard with a F14 key or removing your Excel preferences file.