The third party Android app Tusky has just been updated to version 20 in Google Play.

The new version allows you to follow hashtags within the app itself, instead of having to log in through the website first. ("Click on a hashtag and then on the icon in the toolbar.")

The third party app Fedilab also supports this.

If you are still using the official Android app, you might want to try Tusky or Fedilab instead.

(For F-Droid users, Tusky should be updated there soon.)