3G MiFi --> Airport Express --> Ethernet client possible? Getting error 6753

Hi All,

I am really struggling setting up a friends Airport Express (new N series version).

I told him to buy an Airport Express to allow his Home Cinema Amplifier (Denon AVR 1912) to gain access to his wireless network/internet (via Ethernet cable). This is to use Airplay, as well as the Denon remote app on his iPhone.

His WiFi at home is provided by a Three MiFi device made by Huawei (http://store.three.co.uk/Mobile_Broadband/MiFi)

His PC connects to the WiFi generated by this unit using an N series USB dongle fine. His, and my iPhone connect to the MiFi's WiFi ok too.

Using the Airport utility it sees the Airport Express, and I set it up to JOIN an existing wireless network (typing in the correct wifi password), and have 'allow ethernet clients' ticked. This is also named proxySTA mode I have read.

After setting this, the Airport Utility disconnects from his home WiFi, connects to the Airports WiFi, but then after this I constantly get error -6753. Firewall is turned off so I do not feel it is a security issue. I get flashing amber light on the Airport, or if green light it still will not provide his Denon amplifier with an ip address (Denon amplifier set to use DHCP)

The web server application running on the MiFi device shows an ip address given to the Airport Express fine (same ip address as the Airport utility shows)

Taking the airport express to my house I was eventually able to set it up to connect to my WiFi, and have a laptop connected to the Airport Express via an Ethernet cable to provide the laptop with network/internet access...so I know this set-up is definately possible.

Anyway, does anyone know if doing this with one of these MiFi type products is possible with the Airport Express please? I would tell him to buy an Airport Extreme if that is the solution.