Where can I file an actual complaint?

I'm so beyond frustrated with xfinity and would love to know where I can file an official complaint because I'm just one more lousy customer service experience away from taking legal action. I have genuinely never dealt with a more unprofessional and uncoordinated organization/team. 

I was using xfinity wifi and mobile services and cancelled BOTH before I left the country permanently. I even called the customer service line to make sure and I was assured that all my xfinity services were cancelled. I got calls from 2-3 different reps confirming the cancellation. 

However, to my surprise, I have been getting charged for the mobile services despite cancelling in July. I was charged for 2 extra months for mobile services (August and now September). 

Since I am now abroad, I don't have access to the xfinity app (which is useless to begin with) and have to rely on emailing customer service or chatting through messenger. I literally spent 6 hours chatting with customer service today to find that the representative I talked to only cancelled my wifi service and NOT my mobile service which is why I'm getting charged and now I have to reach out and schedule a call with ANOTHER rep to cancel my mobile service. 

Somehow the most stressful part of moving across the globe has been trying to cancel my services with xfinity and that should sum up how frustrating the experience is. Not only have I monetarily suffered because of some customer service rep's incompetence but now I'm having to waste my time and energy sorting this mess out which should have already been dealt with. 

Seriously considering legal action if this isn't resolved in a week.