Cisco 8821 Manual page 47

The 5 GHz channel width can be configured for 20 MHz or 40 MHz if using Cisco 802.11n Access Points and 20 MHz, 40
MHz, or 80 MHz if using Cisco 802.11ac Access Points.
It is recommended to utilize the same channel width for all access points.
If using 2.4 GHz, only channels 1, 6, and 11 should be enabled in the DCA list.
It is recommended to configure the 2.4 GHz channel for 20 MHz even if using Cisco 802.11n Access Points capable of 40 MHz
due to the limited number of channels available in 2.4 GHz.
Cisco Wireless IP Phone 8821 and 8821-EX Wireless LAN Deployment Guide
47