How do I win an appeal with the medicare insurance company for stopping benefits for skilled care?

My Mom has been in a rehab facility since 12/29 after breaking her ankle on 12/19. I have until noon tomorrow to file an appeal with her medicare advantage plan because they have declared that they will only cover her until 1/19 (day after tomorrow). She desperately needs rehab for several reasons. Before her fall, she was only able to transfer from chair-to-chair due to partial paralysis. However, she has not been able to put weight on her foot since her fall (and won't be able to do so for another three weeks). However, due to her partial paralysis, it is imperative that she continue to maintain strength in her arms if she's got any hope of regaining her ability to move to the same degree that she did before. If she loses that, I simply will not be able to handle her at home. I was just wondering if anyone has any suggestions about what to say when I make this appeal tomorrow? Anyone had any experience with this? Thank you!