https://www.cnet.com/news/netgear-orbi-router-apple-homekit-problems/

A firmware update on Netgear's Orbi Wi-Fi routers is causing some people to have problems connecting Apple HomeKit devices. Luckily, Netgear has acknowledged the problem and is offering a quick fix, with a permanent solution on the way.

If you're encountering this problem, head to this page on the Netgear community forum. From there you can find the model of your Orbi router and manually update the firmware. Netgear provides a guide if you need help doing this.