Am I responsible for the dental bill if I was never told about the waiting period?

I signed up for dental insurance during open-enrollment (10 months after my start date). My employer gave me a 12 page pamphlet, which said the dental insurance should cover 80% of restorative fees. 4 months later I had some fillings done and the insurance provider won't cover their portion - they say I have a 6 month waiting period because I signed up during open enrollment. This was documented in a larger insurance pamphlet which the insurance company provided to my employer, but my employer did not provide to me. Am I responsible for paying the entire fee if I was never informed of the waiting period? Thanks for your help!
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Unfortunately, ignorance is rarely a valid excuse.
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