You've got the best tools. It's called a crop feeder and is so you can get the fluid deep in the throat past the opening to their trachea. Did the vet not explain how to do it?
This photo below from a post by
@ferretinmyshoes who is a reptile vet. Go to this thread and look for Message #26 where she gives a very good explanation:
https://www.chameleonforums.com/threads/warning-to-all-chameleon-owners.80201/page-2
Although I have force fed a lot of chameleons I am not the best. I can't seem to get them to gape for me. You want to try to get them to gape at you. Often just gently holding them behind their jaws will cause them to open for you. I'm just not good at doing that. I end up resorting to using something like stiff business card or a credit card that has not sharp edges to try to get them to open up. If you slip the edge of the credit card under their top front teeth, they often will open up. Sometimes I've pulled on their the skin under their jaws, but you have to be really really careful because the bones in the tongue are right there, too. I use a syringe and put it deep in their throat well past the opening to the trachea. The trachea moves forward and back in the throat depending on what it is doing with its mouth and tongue. It won't always be at the front like the picture.
Be careful not to damage the delicate throat and tongue structures when you do put something hard deep in the throat. People have torn holes in the esophogus being too rough. Hope that helped.
