Thank you for your posts and welcome back to our community forums. We're here to help.
I'm sorry to hear you're facing some issues connecting to Wi-Fi whilst using the London Underground. You've said that there's an error but not specified what the error is. Are you saying that an error message appears? If so, when does it appear and what is the error message?
Can you also confirm if this is impacting you on all stops or only certain ones? If you're using a mobile handset on the O2 network you will need to select O2 Wi-Fi from your Wi-Fi settings, sign up and confirm the phone you will be using. When you are on the Underground, open your browser and select O2 from the dropdown list.
Let us know how you get on.