Hi & welcome.
If you right klik on the RULER at the top, you get a dropdown list, at the bottom,there's a selection of what
YOU want displayed, I've got beats,measures,min,secs, you choose wot U like, (right kliking is very informative in reaper!)
PS If you right klik on the Grid Icon (in the main toolbar) you can display whole beats/half beats/quarter beats,etc,,,, download the FREE PDF manual (if you haven't already)it's great!
ENJOY!