do you have the posts locally that you want to see #trends from?
(aka. "there are no global #public posts" discussion)

#mastodon #newbie #tutorial
There are no #public posts. 🥺
Posts are sent only to the people following you (plus your local server community which may be a few people or a lot, it depends on where you are), nobody else.
People wanting to follow you cannot see your old posts either.
The system is basically "subscribers only", that's "federation".