Employer Says I Enrolled In 2026 Medical Coverage After I Tried To Add Newborn To 2025 Plan

I had a baby on December 27 2025. At that time I was covered under my employers 2025 medical plan. I had already waived coverage for 2026 plan year during open enrollment (I was joining my husbands plan in 2026 since he had gotten a new job with better benefits). This is documented and confirmed.
After the baby was born I contacted HR to add my newborn to the 2025 plan. I felt rushed because it was so close to the end of the year. The office was closed so no one got back to me until 2026 and the portal wasn’t reopened until January.

The issue is my employer is saying they interpreted my request to enroll in 2026 coverage, not add the newborn to the 2025 plan. As a result, they said I enrolled in 2026 medical coverage and that they cannot revise it bc the election window has passed. This is costing me approx $15,000 for the years

My position is I was not making a new 2026 election. I was trying to use the newborn qualifying life event to add my Baby to my existing 2025 medical plan and I had already waived 2026 coverage if HR opened or processed the wrong plan year. I am asking for this to be reviewed as an administrative correction not as a late 2026 change.
questions:
does HIPAA special enrollment/newborn QLE protection helper if I contacted HR within the required window after birth?
if HR says no such thing as an administrative. Correction what formal process should I request?
Is this something I should escalate to the plant administrator of benefits, Broker, Aetna eligibility, team or Department of labor?

I have documentation showing the Baby‘s birthdate my 2026 waiver and my communications with HR. I’m trying to understand what rights/processes apply before i respond further.
Location: NYC based company