Push and hold the "F" button on the remote and press mute. Release then press 6688.
APEX GT-2711
Enter the pass code of 1225 to clear.
APEX PF-2020
Push display for 10 seconds and enter 3308.
UNIVERSAL REMOTE CONTROL SETUP CODES FOR APEX
RCA 403R - 0748, 0765
RCA D770 - 1236, 1240, 1241
RCA 300U - 023, 032
RCA RCR 311W - 1137, 1141, 1235
RCA RCR 312W - 10156, 10748, 10879, 10765
RCA RCR 412BN - 10156, 10748, 10879, 10765
RCA RCU 300 - 023, 032