The MAC of your Wii is set by the WiFi card in the device, not the OS, and you can't change that apart from changing out the wifi card itself.
However, it's very unlikely that your ISP blocked a local device in your home network. Open up the settings in your router and double check that you didn't accidentally add it to the block list yourself, if you find it there remove it.
Though more likely than that is that your router is not using the old WAP2 standard on the 2.4ghz network anymore, and is instead using WPA3, which the Wii doesn't support.
There are a few options that you could check to use. If you can configure a guest WiFi point and you aren't using that for anything, you could set the security on that one to WAP2, WAP, or WEP and connect your Wii to that, then disable once you're done playing. Alternatively you can use a secondary, older router and set it up to connect to that one instead of your modern one. That's generally called a gateway and is what I'm using, should take about 5 minutes total and is great for all sorts of retro consoles and devices. The way to set it up is generally a little different for every router but if you Google the name model of your old router and "gateway setup" you should find a tutorial. All you'd need in addition to that is an ethernet cable to put between your old and new router.