Wireless issue with ASUS RT-AC86u router

I found my wireless connection really slow today. My Wi-Fi speed is fluctuating like crazy and speed is really slow. I called my broadband provider, Plusnet, to see if there were any issues with their fibre broadband service. They did a line check and said that the service was fine but they detected there was a Wi-Fi channel issue with my router.

Plusnet suggested that I change my Wi-Fi channel on the router to overcome the issue I am having.

Can anyone advise what I can do to resolve...

Wireless issue with ASUS RT-AC86u router