where to file complaint

Does anyone have information on how to file a complaint for Xfinity Mobile (XM)? I requested a Samsung A50 from XM in April, and unfortunately the phone arrived with a hairline crack across the screen. I called them the same day I received the phone to report the damage and spoke to XM rep Rudy [EDIT:EID].  He ordered a replacement that same day, (same phone), told me the $35 re-stocking fee would be waived, which it should be because I received damaged merchandise, told me how to return the phone to XM, asked for my consent to approve the exchange and told me the order went through. There was no call reference # that he could give me to refer to later, only his ID#. 

Fedex came to my house on 4/20/20 at 5:53 PM to pick up the defective phone (per the tracking number it was delivered to XM 4/29/20 at 10:15 AM).

When the new phone arrived there was a problem and a rep named LaBarbara assisted with activating the replacement phone. 

I foolishly thought all was well until today, 5/04/20, when I received an email from XM stating that my return had been reviewed, and I later saw a charge on my credit card for $350.00 for the defective phone!!!

I immediately started an online chat with XM on 5/04/20 at 10:00AM, with a rep named Deborah who was in Canada. After holding with her for one hour and 28 minutes while she escalated things to Tier 2, she advised she could not resolve my problem, and that I would have to call Account Verification at 844-963-0011, which I did.

My first connection with a rep after holding was horrible, and I had to hang up, call back and wait in the queue again.  This time I opted for a call back as the length of the queue had grown. I received a call back from a rep named Rachel, who advised that even though I called the correct number, she was in fact "Customer Service", but as far as she could tell I was charged as if I had damaged the phone.

Rachel transferred me to another queue, and after being on hold for 52 minutes, I spoke with Byron [EDIT:EID]. He advised he would have to file a claim with Fedex to have the case "researched", that this would take from 14 to 21 days, that I would not recieve any kind of contact, but "hopefully" things would be resolved and the charge could then be reversed. Not finding this acceptable, I asked for a supervisor, and was told there was not one who could give me a different resolution. Basically, XM's Tier 3 would work with Fedex to resolve, and there was no way for me as the customer to directly escalate this problem. I asked him what would happen if Fedex denied damaging, which could in fact be true, it could have been damaged before it ever left XM, he had no response other than leaving me to undderstand that the charge would remain. This being completely unacceptable, to be charged for a phone I did not damage with no way of reversing the charge, all he could offer me was a case number for the claim filed with Fedex.  This is not customer service, and I need to know how to escalate this on my own- anyone have any info on how to do that?