How to setup the secondary paper tray for a RU-518? It shows 400gsm capable.

Our relay unit is working but its not giving option for using the secondary paper tray. We updated firmware to latest C6100 version.

Relay Unit RU-518

See Paper Weight: "Straight output, secondary (sub) tray output: 40 g/m2 to 400 g/m2 / 11 lb Bond to 106 lb Bond"