https://support.apple.com/bg-bg/HT202068

2.4GHz channel width
Channel width controls how large of a "pipe" 'is available to transfer data. However, larger channels are more subject to interference and more likely to interfere with other devices. A 40MHz channel is sometimes called a wide channel, and a 20MHz channel is a narrow channel.

Set to: 20MHz

Use 20MHz channels in the 2.4GHz band. Using 40MHz channels in the 2.4GHz band can cause performance and reliability issues with your network, especially in the presence of other Wi-Fi networks and other 2.4GHz devices. A 40MHz channel might also cause interference and issues with other devices that use this band, such as Bluetooth devices, cordless phones, and neighboring Wi-Fi networks. Routers that don't support 40MHz channels in the 2.4GHz band do support 20MHz channels.



5GHz channel width
Channel width controls how large of a "pipe" is available to transfer data. Larger channels are more susceptible to interference, and more likely to interfere with other devices. Interference is less of an issue in the 5GHz band than in the 2.4GHz band. A 40MHz channel is sometimes called a wide channel, and a 20MHz channel is a narrow channel.

Set to:
For 802.11n access points, set the 5GHz band to 20MHz and 40MHz.
For 802.11ac access points, set the 5GHz band to 20MHz, 40MHz, and 80MHz.

For best performance and reliability, enable support for all channel widths. This allows devices to use the largest width they support, which results in optimal performance and compatibility. Not all client devices support 40MHz channels, so don't enable 40MHz-only mode. Devices that support only 20MHz channels can't connect to a Wi-Fi router in 40MHz-only mode. Similarly, don't enable 80MHz-only mode, or only clients capable of 802.11ac will be able to connect. Routers that don't support 40MHz or 80MHz channels do support 20MHz channels.
You would..