Which LTE modems can be used if there is no network on available?

In principle, you can use all common models. The following are models that have already found application: 

Manufacturer	Model	Link
TP-Link	tl-wr902ac	https://www.tp-link.com/de/home-networking/wifi-router/tl-wr902ac/
Vodafone	gigacube	https://zuhauseplus.vodafone.de/gigacube/