Promotional Credit denied

I purchased a Google Pixel phone through an online order that has a $600 promotional credit upon a valid trade-in.  My order was canceled the next day when I went through the drivers license verification process because the promotion was only valid until the prior day but the order confirmation I received said I had 36 hours to verify my identity.  I called customer support and she acknowledged the issue and placed another order taking my trade-in information and assured me I would receive the promotional credit. I received the Pixel phone a few days later with no instructions on how to trade-in my old phone.  I called Xfinity and they said that the promotional credit was no longer available.  I said I did not want the phone then and he said be rest assured that he will raise a ticket and my promotional credit would be applied.  I called the next month when the credit did not appear and was told they were going to put in another ticket as the first guy did not do so.  I called various times over the next couple weeks trying to get resolution to my issue.  The agent finally assured me that the credit would be applied and added a credit to my account and told me that I did not need to call again.  After not receiving the credit again after a few months I chatted with someone last week who assured me the credit would be applied.  I called today and was told that they would go back and listen to the recordings from when I ordered the phone and activated it.  I then received a call from a rude supervisor who told me that they have no way of pulling the call recordings as they only keep them 40 days.  He also said that they sent me emails that I have no record of but I have record of all the rest of my Xfinity communications.  I asked him to send me a copy the email that he claims they sent me and he said he cannot as they are generated and cannot be resent.  I asked to speak to his supervisor and he told me he was the top of the line and hung up on me.  I am shocked how rude he was.  I am also shocked by all the assurances I received from the various Xfinity mobile agents that the supervisor told me they had no record of.  My lengthy conversations with agents resulted in one line of notes.  There is something very lacking with Xfinity mobile's customer service.  I am a platinum customer and never have been treated like this on the cable/internet side.