I think this is the main reason developers should always be designing their software with local-first (aka offline-first) approach in-spite of the Internet and the cloud [1].
Regarding the cellular based authentication, it is perfectly doable to be securely authenticated even without the any connectivity and there is a solution to this based on combination of MFA and OTP [2].
[1] Martin Kleppmann talk on local-first (LoFi):
https://news.ycombinator.com/item?id=39444519
[2] A lightweight and secure online/offline cross-domain authentication scheme for VANET systems in Industrial IoT: