Dead characters on iMac Mojave, NOT keyboard-related 2002 iMac running Mojave with Apple wireless keyboard

Today, a cluster of characters on the right side of the keyboard (7 8 9 u i o j k l m .) quit working, both caps and lowercase. 

To narrow down the problem, I replaced the keyboard with a spare and re-booted. The issue remained, leading me to believe it's not a keyboard problem.

I enabled the keyboard viewer and all keys work when clicked.

SOLVED — The problem was inadvertently enabling Mouse Keys.
Sounds like you have accidentally activated Mouse Keys.  Turn it off

https://support.apple.com/kb/PH18381