Hi there,
I have a 1520.3 on AT&T, just upgraded to 8.1. After upgrading, the wireless tethering did not work. Many steps were taken, but none worked. Prior to 8.1, I was able to use the installation of the Spanish language keyboard to enable tethering. After 8.1, it never worked.
I called at&t, who said that Windows phones "use a completely different way to access the internet as opposed to iPhones". He then mentioned that my settings were still set up as an iPhone account, not a Windows Phone. He switched the settings and I restarted my phone with a power cycle. As soon as the phone restarted, internet sharing / tethering worked perfectly.
I do not have unlimited service, but rather I have one of the newer mobile share plans with tethering built in.
I hope this information is valuable to everyone.
Best regards,
Eric