Is there an attorney in the state of MO that can help with a Medicare/DME/ABN/Noridian issue?

I'm not even sure where to start so I apologize in advance if I skip around a little. My name is Tara and I have been trying to help my father with an issue dealing with Medicare and his prior DME company. My father has COPD as well as other health issues. He is currently on hospice. To make a long story short, we have had issues with Medicare denying payment for his oxygen concentrator. So, I filed an appeal and received a response from Noridian. Noridian found that since no boxes were checked on his ABN, that his DME company was responsible for the charges. He is now being billed again for all of those charges even though it stated in the letter from Noridian that he was not responsible. He also received an EOB from Medicare that he may be billed for over $11,000. There is more to this story but no room to type it. I also caught the DME in a lie about his ABN on a recorded line. Please let me know if this is even worth pursing.