Law for insuring in between a medical treatment?

I was undergoing my pregnancy treatment, starting from May 2021. During that time, I was insured under German insurance, not Dutch insurance. My hospital used to send me invoices quarterly, which I used to pay from my pocket. From November 16, 2021, I was insured under Dutch insurance. But my hospital send me an invoice(with only the total amount mentioned, not the detailed invoice) from September 2021 to December 2021 for me to pay. I requested them to break the invoice from September to November 16,2021 for me to pay out of my pocket. And another part of the invoice from November 16,2021 to December 2021 to my Dutch insurance company so that they can pay on my behalf. On this the financier company of hospital told that "I have to be insured in Dutch from the start of my treatment, only then the invoice will be sent to my insurance company. Since my Dutch insurance was started during the middle of my treatment, I have to pay out of my pocket even though I am insured in that period". They also stated that the above statement is a Dutch government law. If I don't pay the full amount till December, they will start legal action against me since I am going against the Dutch law. I need help here to know whether there is any such Dutch government law which says that I have to be insured in Dutch from start of my treatment in Netherlands. In case I am insured in the middle of my treatment, invoices cannot be broken down and send to my insurer for payment. Please help me to provide the advice on this and let me know if you need more information.