https://lemmy.world/c/newcommunities Is probably a good start to advertise it initially.
Also be prepared to work on the community for a long time. Think of it like tending to a garden. Lemmy is still pretty new as a platform but it is obviously growing. Some niche topics are less popular and will take even more time to grow compared to other topics. If you're willing to continue posting and keep the community productive eventually more people are going to join.