Insurance denied toddler’s emergency room visit

We took our (then) one year old to the emergency room last December for RSV. We didn’t know he had it at the time he was just very sick so naturally that is what any parent would do. We aren’t the type that will just go to the ER. He was really sick : crying with nose clogged, coughing, having trouble sleeping etc.

We just got a bill now 8 months later for $500. We thought it was already taken care of since we always pay bills on time but upon talking to the provider they said it was because the insurance took a long time with the claim and ended up denying it saying they aren’t responsible. The provider couldn’t give me any reasons why and said we have to talk to the insurance.

My wife no longer has that insurance so I’m hoping that won’t be a problem. I feel like they might say they denied it because it wasn’t a real emergency. Well what do they expect? Wait until he stops breathing, plus what if he had covid?

If they say this then what can I do to try to get them to cover it? Do I have any options? Thank you in advance.