Medical Front Office Convinced Me I’m in network even though I am not – what to do?

My usual obgyn is no longer in network. I had a procedure scheduled with them and my insurance has no out of network coverage.

I had already confirmed with my insurance they were out of network and with a billing person at the practice who told me to bring my insurance card at time of appt where they could “double check with my insurance”.

Day of my procedure I went into the office at front desk, told them I want to self pay which would give me discount. Three separate people looked at my insurance card and said they are in network. I told them of my past conversations with my insurance and a billing person on their end and they told me they bill using a different number or code and even called Aetna while I was there to confirm and told me, yes, they are in network. They looked at this for at least ten minutes and they convinced me they were correct even though I still felt uneasy.

Welp, sure enough, I’m checking my insurance claim and it has been denied as out of network. The bill is almost $800 and if they would have allowed me to self pay as intended it would have been half because they offer a discounted price for self pay.

What are my options? Is it too late now to get a discount. I feel angry and duped.

Edit: Age 51 California