I am not sure which configuration you have:

On the Server connected directly to UPS via COM/USB port:
1. On WebGUI go to Setup->Hardware->UPS settings function,
2. Set "UPS vendor" to APC,
3. Set mode to "single" (there will be no need to set Master and enter slave IP), 4. Set cable and UPS to match your UPS model,
5. Apply changes,
6. Go to Status->Hardware->UPS status and check if UPS work properly (please disable power from UPS for test).

On the Servers working as APC clients:
1. On WebGUI go to Setup->Hardware->UPS settings function,
2. Set "UPS vendor" to APC,
3. Set mode to "Slave",
4. Enter IP of the "Single"configured server,
5. Set port to 3551,
6. Apply changes,
7. Go to Status->Hardware->UPS status and check if "UPS Mode" is set to "Stand
Alone" or "Net Master" and status is "ONLINE".