WorkGroup Bridge (Wi-Fi Repeater) mode doesn't work on Cisco WAP121
Cisco WAP121 Wi-Fi access points do not work properly as wireless repeaters (extending signal from a Hitron cable modem/router). WAP121 actually establishes connection to the main router and can be used as a bridge for wired clients. However, as soon as you try to enable Access Point Interface, connection to the main router drops...
Cisco calls this WiFi repeater mode "WorkGroup Bridge".