Retroactively Dropped and Denied Coverage

I had a baby and shortly after relocated out of my coverage area in April or so. Despite my best efforts to get ahead of the paperwork and not create confusion, the whole thing turned into a mess.

First, my employer did not record my request to change companies until early August. When they did, they made it effective May 2024. Since I have a newborn, we’ve been attending various appointments in between.

I received three letters from my previous insurance company in the following order:

The first let me know they’d drop me as of 7/31

The second let me know they’d drop me as of 8/31 (This one I cannot find)

The last one let me know they’d drop me as of 5/31

I took my baby to an appointment on 8/14 and they let me know they are not covering it via a rejection letter dated 8/16.

Can I do anything about this without that missing letter? Even if I find it, can I do something about it? What about other items that took place between 5/31 and now- can they require me to refund it?

My new insurance does not cover the office I took my son to unfortunately.