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I think I've solve the problem, in my case at least.
WORKING! Connect Wii to Internet using Mobile Broadband Connection
WORKING! Connect Wii to Internet using USB Modem
Also applies to PSP/DS/DSi/Xbox360
On the host computer, follow these steps to share the Internet connection:
1.Log on to the host computer as Administrator or as Owner.
2.Click Start, and then click Control Panel.
3.Click Network and Internet Connections.
4.Click Network Connections.
5.Right-click the connection that you use to connect to the Internet ( In my case the Vodafone Mobile Connection ). For example, if you connect to the Internet by using a modem, right-click the connection that you want under Dial-up.
7.Click the Advanced tab.
8.Under Internet Connection Sharing, select the Allow other network users to connect through this computer's Internet connection check box.
9.If you are sharing a dial-up Internet connection, select the Establish a dial-up connection whenever a computer on my network attempts to access the Internet check box if you want to permit your computer to automatically connect to the Internet.
10.Click OK. You receive the following message: When Internet Connection Sharing is enabled, your LAN adapter will be set to use IP
address 192.168.0.1. Your computer may lose connectivity with other computers on your network. If these other computers have static IP addresses, it is a good idea to set them to obtain their IP addresses automatically. Are you sure you want to enable Internet Connection Sharing?
The instructions above are good for the Host side. The only part I have different is the Establish a dial-up connection whenever a computer on my network attempts to access the Internet is not checked on mine, I dial up on the PC before attempting connection on the Wii but it might work for you.
Also, ensure that under 'Home Networking Connection' that 'Local Area Connection' is selected ( this is the wired connection to you router).
( UPDATE: automatic dial didn't work for me so I just manually do it before trying to connect Wii/PSP/DS/Xbox360 to internet )
On the Wii side "automatic detection" is not good. Has worked but is very hit and miss, once in a blue moon for me, so I set the values manually.
192.168.0.2 ( Last number can be any unique number in range 2 - 255 )
192.168.0.1 ( Must be this when sharing internet connection )
I have a Linksys WRT54GS v5.1 with default settings ( return to factory defaults by holding the reset button for about 20 seconds ). Hence no firewall or security settings.
Connects 99.9% of the time now. The 0.1% could be just poor signal on my Vodafone Mobile Broadband dongle. I even connected my PSP using the same method. Works fine. Also my NDS can now go online with the above method. So too can my Xbox360, although this is wired to my router and so DHCP seems to work ok for it too.
As a sanity check I hooked up another old wireless router, a ZyXEL P-660HW-D1 instead of the Linksys, also reset to factor defaults. This works fine too.
Happy Days. Next step is to enable WEP/WPA security and try it out but I don't expect any issues here.
Hope this helps some people.
UPDATE: WEP working ok. WPA working ok ( NDS doesn't support this ).
Did try WPA2 but PSP doesn't support this. So WEP is needed to cover all consoles.
Forgot to say how I originally got the above settings. When I wired my Xbox360 to the router it correctly setup automatically with those settings so tried them on the Wii ( with unique IP address ). It was the 'Primary DNS' setting that I had been missing on previous attempts, I had just left this blank.
NOTE: if you need to connect to your router then you need to uncheck the box 'Allow other network users to connect through this computer's Internet connection' and disconnect from the internet. Otherwise, for me anyway I cannot connect to my router to change security settings etc. Just re-check the box before dialing back up and trying to connect your console to the internet again.
My Wii v4.2E. Tried on v4.1E also works.