Duo: No WiFi? No Cell Service? No Problem
Even if you have no access to WiFi or cellular service, you can still use Duo Security.
Use the Duo Mobile App
You can use the Duo Mobile app to generate bypass codes while you're traveling internationally, in Airplane Mode, in a basement laboratory, or on a mountaintop. Follow instructions for Duo: Use the Duo Mobile App.
Generate Duo Bypass Codes in Advance
If you don't use the Duo Mobile app, be prepared with Duo: Generate and Use a Bypass Code.
You can use bypass codes whenever you need to sign in where you have no WiFi or cell service. The codes are good for one year (365 days), so even if you are planning on spending a lot of time "off the grid," you will be able to access online systems, data, and assets.
Here is how to add Duo to a new phone.