Hello,
You can modify the allowed channels for this scenario using the channel command under the radio interface view. For more details, you can check the following guide: channel (for FAT AP).
Please note the following recommendations:
The channels you configure must be supported by the terminals; otherwise, the terminals cannot discover wireless signals. For example, when the country code is set to China, 5 GHz channels 36, 40, 44, 48, 52, 56, 60, and 64 can be configured. However, most terminals do not support these channels currently. If these channels are configured, the terminals cannot discover wireless signals. In this case, you can configure 5 GHz channels 149, 153, 157, 161, and 165, which are supported by the terminals.
And if you want the channel unchanged, you should disable the calibration with the command undo calibrate enable under the WLAN view. For more information please refer to: calibrate enable { auto | manual | schedule time } (for FAT AP)
Please monitor if the channels keep changing after this as well.