As above check the manual, and possibly router itself (most will have a sticker on, might need to reset everything as first login can see people change things).
802.11b is not really a setting on things, though I have seen some use it. It is going to be more of a sequence of settings (no security, WEP or for the Wii maybe WPA I think it allowed), and can have knock on effects if legacy things are introduced for your other devices (if it is using one of its transmitters to do legacy stuff then it is not available to boost speed of the good stuff, whether this is relevant on the average connection is a different matter).
For this sort of thing we tend to point people in the direction of their nearest junk shop, electronics recycler, yard sale, garage sale, flea market, car boot sale, online tat site... basically wherever people go to sell old things. Buy a router of the time from such a thing (everywhere I go they are commonly available), connect it via a LAN cable, set it up properly (or just disable DCHP and let the host do it, you might have to change the router's IP to not conflict with other devices and use the same gateway). Accordingly it is generally advised to avoid routers sent out by ISPs as they are usually locked down and prevented from doing anything fun, though occasionally you might find them being so popular means they became targets for custom firmwares.
Alternatively set up a wireless hotspot on your computer, or possibly an old phone, and set that up accordingly, though modern windows might make it hard to use more legacy feature sets.
Alternatively the wii had a USB to LAN cable option. Could be the easier route in this, though a search just now says they have since become a desirable device, possibly for this reason, so you might have to pay up for it (not quite the silly money of rare RPG collecting type scenarios but enough that I note it in this and will continue to add them to the pile when I see them in junk shops).
