After working in the medical field for several years, I've learned that insurance companies are very good at hiding all the fine print to the policies they sell, especially those to self-insured people who aren't exactly sure what they are looking for. Most of the time these policies are very expensive with outrageous deductibles: our office has seen patients that have paid over $500 in monthly premiums and the policy has a $5000 or more deductible and then only cover at 50%.
There are a few things you can do. You need to check your policy and see if it states a specific exclusion to a specific health condition. You mentioned it has a rider, does it apply to your diagnosis and treatment in any way? Did you have any health insurance coverage at all prior to the effective date of you policy? With some insurance companies, if you were insured by another company and have a letter of credible coverage, and you can prove no lapse in coverage, sometimes the pre-existing can be waived. But, some companies do not care and will enforce the pre-existing regardless of prior coverage.
If you weren't insured at all before this new insurance and you have a pre-existing on you policy with a waiting period, most of the time the insurance company doesn't care-- it's as though they have a year to collect your premiums without having to pay out for any claims which is how many of the smaller companies make the most of their profit.
If your appointment was routine and was only scheduled in June, but you didn't see the doctor until the day your policy took effect in November, the medical records in your office should reflect that fact and could be used to establish diagnosis after the policy was effective. But, sometimes these companies do not care and if they stated a one year waiting period, that's exactly what they will make you wait regardless of diagnosis and policy effective date.
You had said your doctor's office called the insurance company, verified coverage and was told pre-existing would not apply. Most insurance companies assign reference numbers to all calls and if that is the case with your company, this call and all the information they gave your doctors office, including the statement of your pre-existing not applying to this specific problem, should've been recorded and given a reference number. Ask your doctors office if they have that reference number and if so, the office needs to use that as one way to try to appeal the denial. Your doctor's office should be trying to help you in some way since their claims are being denied.
I checked about the insurance commissioner in your state and I found this link. At the bottom of the page is a number to call for help with claim disputes. If you and your doctors office can prove they were told your pre-existing would not apply and that the office had all its duck's in a row, this may be an option and resource that can help you. I can tell you, when an insurance company has the threat of being turned in to the states insurance commissioner, it's amazing what can be accomplished.
I hope this was of some help to you. I'm sorry that you are going through this and I wish you the best of luck. If you have any other questions, please feel free to PM me.