I assume that DUO3 is a modchip(which I have never heard about it). Well, I have another modchip called TPIIG (Thunder Pro II Gold Edition) installed on my PS2 Slim (SCPH-90006 HK BIOS 2.30) console. The network only works when the DVD+R DL fix (from my modchip built-in menu) is enabled. The TPIIG manufacturer confirmed that network modules are enabled together with DVD+R DL fix (don't ask me why they implemmented this way :-) Maybe you should try the same in case your modchip has a similar built-in feature like mine (this shouldn't hurt). btw, take a look into my recents posts about SMS, OPL and my Nano Digitus NAS DN-7023.
doctorxyz's PS2 & PS3 stuff: (http://psx-scene.com/forums/f257/doctorxyzs-ps2-ps3-stuff-101348/)