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".
The issue was resolved by upgrading WAP121 firmware from 18.104.22.168 to 22.214.171.124.