I need help resolving an issue

I have a mobile billing issue. I have called Xfinity 7 times. I have requested a supervisor 4 times, each time there was a reason I could not speak to someone, yet they promised to call me back. Never got a call. 

my issue is I am the daughter, POA for my mother with dementia. I have been paying her bills for 3 years. Never before, and never since has she gone over her mobile data. In June, she was billed $497. 00 and some change for overuse. She has dementia, cannot drive, and does not leave her assisted living apartment. She has Xfinity WiFi there . They cannot explain to me why she would accrue this debt. There was no power issues,( per her nurse,)  and her phone is set to WiFi. She cannot even find the volume button much less change her settings. She is not responsible for this bill. The one rep, said they would give a 200$ credit, never applied, another said to wait for the August 1st bill and the charge would be discounted. ( No discount ! I cannot continue to waste hours of my time on the phone with zero resolution.  

One supervisor called my demented mother, and got her more confused. I had mentioned she cannot make decisions and won’t be aware of the issue due to her memory loss/general confusion.  I need this fixed ASAP, .  I am actually in shock at the poor customer service. I am a Verizon customer and have never experienced this with them in 20 plus years. 

I need this issue resolved so I can pay her bill and get it up to date.