For single-user Mastodon instances, it really helps to become mutuals (you both follow each other) with some people on a few big instances. That will establish federation between your instance and theirs. Other things that can help are:
- Interact (like, boost, reply) with interesting posts from another instance
- Fill out your profile! That side of fedi tends to distrust blank profiles.
- Write and sticky an introduction post, for the same reason as above.