My primary health provider has always been in network and I’ve paid a small co-pay for my appointments. I now got slapped with 2 large bills from insurance claiming they paid $0 because the doc is out of network. This is MID YEAR. I was never given notification or warning. Is that legal?

I just got a $300 dollar bill from my insurance and then found yet another $178 bill in my bill history (that one was never sent to me directly). Both claim that my provider is out of network. I have been going there 2 years they ARE (or were?) in network. I was never informed or warned by ANYONE about any type of network change.

Who is responsible for this? The medical provider? Are they allowed to make a change like this mid-year?? Are they allowed to make that change without warning?

Is the insurance company responsible? Was there any communication between the two?

Who can I write to appeal this? I already messaged my healthcare provider and am planning to send an appeal to insurance but need to find out what they say first.

EDIT TO ADD: This medical provider is a paid subscription. I pay yearly and can meet with any doctor in the office, and will only pay a standard co-pay because the whole business is covered in network. My paid subscription runs through next February, so if they changed accepted insurances mid-year, that seems like it would be illegal to me.