Open enrollment vs new employee enrollment..

I was hired in late October with my company and eligible for health insurance after one month. Open enrollment for all employees was in full swing when I became eligible. I followed all of the instructions sent to me for enrollment and there was nothing to indicate that there were separate enrollment paths for new employees vs. regular employees. I enrolled like everyone else and I received benefits for the month of December. Insurance cards were sent to me. I scheduled a necessary dental surgery in January and was told by the dental office that I had no coverage. I went to HR to inquire and they came back to inform me that I had only signed up for benefits for the last month of 2023. I would like to know if anyone knows how this could happen and if I have any recourse at this point. Meaning that even if I can prove this was not my error, will my company have the ability to sign me up at this point for 24? I would appreciate any information. I can't be the only person who has become eligible for new employee coincidentally during open enrollment

Thank you