It isn't about "who are you with?" Or " do you have a modeling agency," it is about finding the right agency or agencies for you.
Obviously fashion agencies want a girl of a certain height, but for a commercial print model you have more just the world wide agencies to choose from. There are hundreds, thousands of modeling agencies, working with hundreds and thousands of girls.
TV Shows can sometimes give an illusion of what it means to be a model and work in the industry with agencies.
Working as a model doesn't involve being stuck to one sole modeling agency, especially if you are a new model, I suggest freelancing and building your portfolio and experience with many agencies, and then approaching larger and more mainstream modeling agencies and talent agencies. It is easy to market yourself with some experience.
When I started modeling I was freelancing with about 5 agencies.
To build your experience I would mail your photos out to as many commercial print modeling agencies, print modeling agencies are more likely to take on a girl of any height, compared to a fashion based agency. Submit out your cards, photos, and keep mailing until you hear back. A persistant person is more likely to get an agency than just someone who submits a few photos and gives up based on that one mailing.
You can freelancing with many modeling agencies at once and when you are starting you SHOULD. Many girls think that just having one agency is enough. They think that you will get everything, and starting modeling and rolling in jobs with just one agency, and this is just not true, not for most models out there. Many models freelance, it is a good idea because of a few things:
Certain agencies a good for certain things, one agency might be more commercial based, and if you go into PARTS modeling for example, you might want to work with both, and you can without signing an exclusive contract. You can be your own sole proprietor, take care of your own taxes, by freelancing.
Similar to freelance writers and designers, you work with certain establishments and you work on a freelance bases as a model too. Just be sure to give credit always to the agency that sends you out on the casting. Respecting the agents you work with is important to be sent out again and again for jobs, and castings.
You should be working with many agencies when you are building your portfolio, and modeling experience. Modeling isn't just something you sign up for one day and let someone else control, you have to be able to run your own business of yourself and working with modeling agencies means not just finding the right one, but when you are a new model, finding the right ones.