In PT for bad ankle sprain, but just badly reinjured getting back to cheer (with a good brace). If PT isn't helping, what's next?

Don't quit PT. Physical therapy will help improve your proprioception so as to improve your ability to prevent future ankle sprains. Even the best brace can't prevent all injuries. I understand you are eager to return to cheer, but patience will be your best friend.
RICE. Rest, ice, elevation of that limb. To facilitate healing it needs time to heal just like anything else. Continuing to exert on that limb will cause reinjury and eventually the possibility of permanent injury. Do not ice for more than 15 minutes at a time as this can cause damage to your skin, do not apply ice directly to the skin either, wrap it in a towel or something of that sort.