Trade Ins, Promotions & Billing

I was wondering if there's a formal line to make a complaint. Back in September I had added a new Premium Unlimited line, initiated a trade in and for both of these lines purchased the new IPhone 17s. It's been a headache of being overcharged since then with no resolution. They received my trade in but didn't apply any credits to my line and never added the $400 promotion for my new unlimited line. I call in every month and get the same story, we hear you, we see the issue, we've corrected it - look out for an email with the resolution. I have not received anything and I continue to get overcharged and with my autopay, continue to overpay. I even received a rep who opened a ticket for me and said expected timeline for resolution was 2 weeks but here I am 4 weeks later, overcharged again. I'm at the point that I'm considering ending my services with Xfinity: cable, internet and mobile.