I did for years. I was working in gyms in college, cleaning equipment and checking people in. Then got some shitty cert and started working for Bally. Even with like 3 years experience I had more experience then 90% of their trainers, aka most larger chains have shit trainers.
I did better in private settings and in person. After awhile of doing it, I was charging like $100 an hour and you can def make good money with a good clientele. But I find that you really need to have a way to stand out and be unique and special.
We also have THUMBTACK. ITs an app and you can advertise your services and reviews on there and it can do well.
It is a pain in the ass, but you can make good side money.
I would train at gyms all over and I would pre negotiate a fee. Many of the gyms thought they were gonna get 50%. Thats common. But that's if you allow it. I would have the clients join there and also I would pay them a fee, but it was never 50%. The ones that didnt negotiate, and some didn't. I said fine, I will find another gym down the road to take my 30%. And I always would.
That would be my advice. Or align with a good gym, Starting at a shitty gym may get your foot in the door like club24 etc, etc.
You can even be fat and useless and be a trainer at Planet Fitness. They will say wow my trainer is phenomenal. He has like 40% bodyfat levels. He can max out 2 plates on the smith machine bench press. He is worth the $75 dollars I pay an hour even if he has only been working out for 3-4 weeks longer then me