The 2FA providers that come with Extended Two Factor are disabled by default. Go to your Admin Control Panel -> Setup -> Service Providers -> 2FA Providers to list the 2FA providers. To enable or disable a provider, toggle the slider next to the provider's ID. Providers that come with Extended Two Factor are prefixed with 'netf_'. The IDs for each provider are listed below.
Security Key (U2F) 2FA Method
Provider ID: netf_fidoU2f
As the add-on requirements state, you must ensure that your board URL in "Basic board information" within the XF Options is the HTTPS version of your URL. Also ensure that your URL has no trailing slash on the end, e.g. your URL should look like "https://forums.nanocode.io" and not "https://forums.nanocode.io/".
Note that if you change your board URL you may run into issues with existing security key authentication devices (validations may not confirm). In some rare cases pressing the security key button immediately after logging back in will not work; in these cases, the user should click 'Continue' manually (which shows the "two-step could not be confirmed message") then try again, pressing their security key's button.
SMS (Twilio) 2FA Method
Provider ID: netf_sms
To use SMS 2FA, you must create a Twilio account. After setting up your account, you will need your Account SID, Auth Token and a number you wish the 2FA texts to be sent from. You can find your Account SID and Auth Token in the Twilio console. You can view or add more from numbers in the here, in the Twilio console.
After you have these pieces of information, navigate to your Admin Control Panel -> Options -> [n] Extended Two Factor. Input the information into the respective text boxes on this page.
Please note that SMS 2FA via Twilio is not free. You can view Twilio's pricing here.