Trouble connecting to XFINITY wifi, using Kubuntu 20.04
I had the same problem and your question led me on a wild goose chase to figure out GTC inner authentication with KDE. I tried PEAP+GTC using KDE Plasma network manager but once the connection is established, you can't get past the network login prompt / Xfinity wifi connection instructions page.
The solution I stumbled into is to use TTLS Authentication + PAP inner authentication. The network now connects without any network login prompts.
TLDR:
Use these settings and fill in your username and password:
