Under Tools - Options - General - Input Settings, there is a checkbox for Press Enter to Move Selection Down. But it doesn’t accomplish what I need: to enter a value in a cell, then have the cursor go to the next cell down.
What does happen when you press Enter with that option checked and with Down selected on the dropdown?
Nothing. The cursor remains where it is.
Makes no sense to me. The default is to move the cell cursor to the next cell below after one closes input with Enter and that works about ~everywhere. If it doesn’t work for you, then check that your Enter key is actually an Enter key… 
i.e. not redefined to send some different key code.
Thank you. Now that I know it has been set to the default, I will look for other explanations such as you suggest. :>
Go to Tools, Customize, Keyboard.
Select Enter, and then Reset.
The default value is Move Down.
Hallelujah!