After deploying a 5 GHz-only WLAN infrastructure in the USA, using 20 MHz channel widths...

This issue arises due to partial support for 5 GHz channels by some client devices. Although the infrastructure may use all 25 available 5 GHz channels in the U.S. (spanning UNII-1 through UNII-3 and DFS channels), many older 802.11n tablets and clients only support a subset of these channels—commonly the lower UNII-1 band (channels 36–48). As a result, even though the APs are operational and signal coverage is valid overall, the client may not see or associate with APs operating on unsupported 5 GHz channels such as channels 52–64 or DFS channels (100–144).

This leads to inconsistent connectivity or lack of signal in areas where APs are broadcasting on unsupported channels. The CWDP-305 Study Guide advises taking client capability into account during design validation and includes best practices for mapping expected device support versus operational channel plans.