You will get this with “More Settings - Network Mode - WDS”.
After going through the wizard, the Mango will have an IP from your main router.
Devices on both LAN Ports will also get IPs from your main router.
To enter MT300N setup screen you must now use this IP.
You now can disable WiFi at “Wireless - On/Off”.
Unfortunately you need the latest firmware 3.203 to use that feature without a bug.
So first update Firmware.
Bug in 3.105 and older: when rebooted or reconnected to power, bridge WiFi will not reconnect if AP WiFi is switsched off. You will have to reset the Mango and repeat the WDS setup after each reboot or repowering. .
Also you might have to switch off “Autochannel Mode” in 2.4GHz if you use a FritzBox, as the Mango will only use the initial WiFi channel from the time when WDS setup was done and not rescan if the main router changes the channel. (not yet accepted as Bug, but thats what I found out with my FritzBox 7530)