How can I find out which channel is least used near my access point?

Zyxel_JoyLee

You could use the function "Neighbor scan" of Nebula Live tools that scans the surrounding Wi-Fi environment.

Site-wide > Devices > Access points > Live tools > Neighbor scan

Neighbor Scan allows the access point to scan nearby wireless networks and show which channels are being used in the surrounding environment. You can use this information to identify a less-used channel and set it as the preferred channel for the access point.