Promo gone wrong

I signed up for Xfinity mobile about 6 months ago. I was told that my monthly payment would be $17.92 because of 2 promotions I took advantage of. I continually get billed for $36.$$ and have called several times. A temporary credit appeared and then right back to the price that should take effect AFTER the promo ends. Today I called to talk to a supervisor. One was SUPPOSED to call me back and has yet to do so. I’m really ticked about this.