Hi, and welcome to the forums.
If you did your vertical zoom on the track control area, yes, you have to click in the arrange area or time line to do horizontal zooms. I got around this by setting it up to allow vertical zooming in the arrange area with ctrl-mousewheel. This way I don't need to click on track control area for zooming at all. Ctrl-wheel = zoom vertical at mouse cursor; mousewheel alone = horizontal zoom.
There are a lot of ways to do anything in Reaper. This is just my approach.
You might want to check out this video for tips on this:
https://www.reaper.fm/videos.php#1_ElH-fMjhc
And the others from Kenny Gioia:
https://www.reaper.fm/videos.php