I've had a C9 for years, and it's been working for the most part. A couple of days ago internet on the 5GHz WiFi stopped working - I can connect, but no internet.

The 2.4GHz connection is working fine, so there is no issue with the internet connection.

This is across multiple devices (laptops, phones, WIFI extenders etc.) - 2.4GHz works but 5GHz doesn't.

I've tried rebooting the router a few times but no joy. Any tips on what to try next or how to troubleshoot?