Billing and iPad service

I need to speak to corporate or someone above managers. I have been dealing with this issue since November when I received a bill over $1700 for international data, ever since then I was put on a repayment plan and Xfinity has been messing up the payments they don’t take them out and I have to go through the process again and again now my service on my iPad is being affected both iPad were reading invalid sims and had to be replaced by Apple bc Xfinity wasn’t able to help. Now after getting two brand new iPads one is acting up again reading the same thing invalid sim this is not an apple problem it’s an Xfinity issue where either you guys are blocking the service. I have been to the Xfinity store more than 2 times per week for the last month and no one is able to help me.