Database and application design to guarantee application availability when internet connection temporary lost

I want to create a web application for restaurants and because of business model reasons it should be a online web application (on the cloud). So a restaurant can have an account on the app and it creates its own menu and adds its waiters and the cook.

The waiters should be able to access the menu and place orders all the time. My main issue which i should decide how to go about is:

"How can i grant fulltime availability to the waiters or the cook even when the internet connection is lost for several seconds to several minutes or even hours during the day"

I was thinking of installing the app in a sever in the local network of the restaurant which takes over the responsibility of the could server when there is no internet connection which means all orders are saved in the DB of the local server. And as soon as the connection is back the local DB is synced to the cloud DB (i was told Postgresql might have plugins supporting this, via on-premise or sth similar). Which means local DB records should be pushed to the cloud DB.

Can someone give me a hint on what tech (open source and no enterprise solutions pls) to use, to accomplish the DB syncing when internet goes on and off.

Am i on the right track or completely off with what i suggested previously?