I am going to make one, and only one, attempt at this.
"Dr. X accepts ABC insurance" and "Dr. X is a participating provider with ABC insurance" do not mean the same thing. All participating providers accept the insurance; not all doctors who accept the insurance are participating providers.
A participating provider has signed a contract with ABC insurance to bill you only within the limits of the insurance policy. A non-participating provider has not signed any such contract and is not bound to honor the insurance policy limits EVEN IF THEY TELL YOU THAT THEY ACCEPT THE INSURANCE.
A participating provider cannot bill you for balances beyond the policy limits. A non-participating provider can, even if they "accept the insurance".
Sounds to me as if your dentist is a non-participating provider.