CPT code mixup in peer to peer

I had surgery several months ago, and have had the same bill re-sent from the surgery center multiple times. Some background – prior to the surgery, the procedure was not pre-approved by my insurance. So the surgeon had a peer to peer call with my insurer. I was assured they agreed on how to classify the surgery and it would be approved. Let’s say they agreed on CLT code 123 (made up). Fast forward a couple months after the procedure. I get a bill for the full amount. Turns out the surgery center sent the wrong CPT code to my insurer and it was denied. I made many phone calls, and it was re-submitted using a different code. Denied again. More phone calls. Insurer provided me with the call notes from the peer to peer saying they agreed on code 123. Called surgeon’s office. They say yes they discussed code 123 but ultimately agreed on 456. No paperwork for that from provider or insurer. I’m being billed for 456 and it’s not approved. So I feel stuck. The procedure was last year, and I had already met my deductible, so my out of pocket cost would be very little. Now I’m getting NO discount and I don’t know what to do. PLUS it’s too late to get reimbursement with my HSA since the tax deadline has passed and I can’t make prior year reimbursements. So I missed out on that benefit too. Should I appeal to insurance to see if they have anything regarding code 456, or might I be in need of an attorney? This is freaking exhausting and stressful.