Dentists’ office overbilled $630 for "upgrades" – Delta insurance says they owe me but aren’t responding, what can I do?

My dentists office did a crown and billed my insurance for a code (D2740 – Crown, porcelain/ceramic) and (D2950 – core buildup)

The accepted fee was $812, insurance paid $448 – my responsibility was $364, but this happened AFTER the procedure was already done (I couldn’t wait for a preauth, my tooth was broken)

They billed me $995 for the whole thing on the day of the procedure, and when insurance came through they only refunded about $150, saying that because they did a “name brand Cerec upgrade” to the material for the tooth, that insurance was actually not correct about the costs and that I wasn’t owed anything more.

I contacted Delta and they have replied with a letter that I was overcharged $631 but that they were unable to get a response from the Dental office (it’s a large PDO chain they aren’t going anywhere, just not replying).

Who do I contact now? Do I just walk in with this letter and ask them to reconsider? If they refuse what are my options?

Thank you very much, this has been a long and terrible process. Any help at all would go a long way.