Complaint

How do I submit a formal complaint? 
I have a phone from years ago I purchased through Xfinity. It is still sim locked despite being paid off. I have tried since Sunday to get this issue resolved. I’ve talked to multiple agents, some which REFUSED to connect me with a supervisor. I was finally able to speak to a supervisor and there is nothing they can do. The ticket is open and I just need to wait in limbo. This is a poor way to run a business. I own this phone and can do nothing with the item I own because of your company. I have been a loyal customer for years, since 2019. Two years ago I canceled my mobile service with you guys because of your horrible customer service and inability to effectively help your customers. I made a mistake not canceling my internet too. That will be the next thing to get done. Xfinity has lost a loyal customer and it’s impossible to file a formal complaint or speak with any management. I do not recommend anyone use this company for any of their services.