Need advice - dental insurance wont pay for last claim for treatment while policy was active

Hi all, if someone can give me advice on this topic it would be much appreciated. I received Invisalign treatment while I had active employer-sponsored dental insurance. The treatment was completed while my policy was still active. I had to quit my job due to extrenuous circumstances a few months ago and now my dentist has told me that the insurance company has denied to pay the last claim of around $700. I don't feel this is fair. How does this typically work? Who do I contact? Can I tell my dentist that this issue is between them and the insurance company/my former employer? I quit my job after treatment was finished and have fully paid what I owe out-of-pocket so far. Should I file an appeal with the insurance company?

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Edit: Thanks for your responses so far. Apparently the reason the insurance company denied the claim is because they "pay in installments" over time.