Got a $4,200 bill 8 months after surgery - is this even legal?

So my wife had knee surgery back in July. We paid everything at the time - copays, the anesthesia bill, all of it. We thought we were done. Then last week out of nowhere we get a collections notice for $4,200 from a "surgical assistant" we've never heard of.

Turns out this person was in the OR but wasn't in-network even though the surgeon and hospital were. We never signed anything authorizing an out-of-network provider and had no idea anyone like that would be involved. The surgery was at an in-network facility.

From what i've been reading this might be covered under the No Surprises Act? But trying to actually figure out how to file a complaint or dispute this is honestly confusing. Our insurer is Cigna. Anyone been through this? Did you get it resolved and how long did it take?