It is always good to do a warm-up cardio but your trainer is right on cardio after weight training. By doing weight before cardio you are using your food that has turn into glycogen for energy to build lean muscle to tone but your body, and then doing cardio when your body is depleted of of glycogen...

Well, if your not achieving any results than take a look at your nutrition as it is 80% of weight loss. I would encourage you to incorporate weights into your workouts too for better weight loss.

I am happy to hear that you are going to start using weight training in your workouts as it is an excellent method for burning fat. Now, I am not saying you will lose weight will just cardio but weight training will make weight loss 2x as fast.