Unfair Dismissal

I am a fitness instructor and my classes were given to someone else because the "manager", wants someone specific.

Now I am left with no work at the specific gym, and there are other instructors with a poor performance who still have their classes, and also had the luxury of going through performance, yet they still have their classes, however I lost my work just because the manager wants someone specific.

I deliver excellent classes, the members love me, and I was already building a client base.

The gym that let me go is Planet Fitness Waterkloof. And they dismissed me over a phone call.

I feel like this is unfair, what should I do?