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.
You can use bypass codes whenever you need to sign in where you have no WiFi or cell service. The codes are good for 120 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.