Dentist Said They Accepted My Insurance, Performed A Procedure Where I Paid The Deductible, Now They're Saying That They Don't Accept My Specific Plan

So I've been going to the same dentist for over a year now, and I knew they accepted my insurance carrier. During open enrollment in December, I switched plans (same carrier, just going from a PPO plan to a DMO plan). I had an appointment at the dentist on January 28th, and provided my updated insurance information via online pre-registration the day before the appointment.
The day comes, I show up, they said they checked the insurance and it's the same group number as last year so it's fine. I did inform them that the plan itself had changed, and put the plan name on the pre registration form as well (unfortunately I don't have a screenshot/saved file of the form). Got a root canal, scheduled a follow up, and was only billed for the deductible, which I paid at the time of service (I do have a receipt).
Today, almost a week later, I get a call from the office. Thinking it was just a confirmation since I have a follow up appointment in 2 days. The receptionist says that I owe them $700 because they don't accept my dental plan. I don't believe it's an issue with the dental provider not accepting a claim, but instead an issue with the dental office not running the insurance before the service was performed and then finding out after the fact that they (as an office/practice) don't accept that plan.
So now they want an additional $700, and I have no idea how to proceed. As far as I'm aware, they would've ran the insurance before the procedure was performed, and I would've had to sign a document agreeing to self pay if it wasn't accepted. And signing a document agreeing to self pay and not bill insurance, is different than agreeing to pay costs that insurance doesn't cover. Is this correct? I've heard about the No Surprises Act but am not sure if this applies here. I did not sign anything agreeing to self pay.
Any advice is appreciated, I thought about contacting my employers benefits department and also the insurance carrier, but I don't know how they could help since this is an issue with the provider themselves. Thanks in advance!
For context, this is in Ohio, USA, and the plan is with Aetna
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