Baylor St Lukes/Cigna Insurance Issue

Did anyone experience an issue with continuity of care between St. Luke’s and Cigna last year?

My father was a patient at St. Luke’s Hospital when they went out of network with Cigna in October 2021. He obtained a continuity of care document at the time and continued to pay in network benefits as billed by St. Luke’s. However now St. Luke’s has turned around and told him they are not honoring those in network benefits and is billing him over $100,000 for the care he received last year. His Cigna patient advocate has been in contact with St. Luke’s numerous times over the last few months but St. Luke’s won’t budge. Any advice on what to do in this situation?