Authenticator apps that use TOTP (all of them) don’t need an internet connection. The auth codes are time-based and generated on the device, online or offline.

Authenticator apps that use TOTP (all of them) don’t need an internet connection. The auth codes are time-based and generated on the device, online or offline. I use authenticator app or hardware key if supported. Adding TOTP 2FA to a web site is easy.