As of 5.4.0, Activity Search gives an empty result set - patch is available
I could replicate this issue on my local machine, this seems to be a core bug and have logged issue at https://lab.civicrm.org/dev/core/issues/353.

I think the search does provide results when we disable record type options


So in the mean time until the bug gets fixed you can rely on this option (hope it shoes correct count of results ;))

I have also submitted PR for this at https://github.com/civicrm/civicrm-core/pull/12713 if you can please try after applying patch and then comment on PR about your QA that would help to speedup the process of merging the code in next release.