United Healthcare says primary is out of network, but primary is listed as in network on their website.

So I went for my free annual check up, then got hit with a $600 bill. United Healthcare said my provider is no longer in network and its my responsibility to make they're in network before visiting them. The problem is the primary is listed as being in network on their website. I submitted an appeal with a screen shot of their website and they denied the appeal saying the supporting documents I submitted are not official. Does anyone have any suggestions as to what I can do in this situation?

UPDATE: I called United Healthcare and a rep reviewed my case and allowed the claims to go through, confirming that everything was in network. This is after a month of calls and appeals, so my guess is they're trying to make people work for it. I almost gave up and just paid it, but thanks to people here letting me know that this is something United Healthcare is doing, I decided to keep fighting it. Thanks to everyone who responded, you all saved me $600 :)