Can I use TPE 3D while offline?

Not at this time.

Maps and elevation data are cached on your device and so may be available while offline, but you should not rely on this.

Clearing the cache from Settings while the device is offline will clear all data and the app will not display any 3D terrain or lighting.

Navigating to a new location, performing location searches by name and time zone determination all require an internet connection at all times.