HTTPS / Certificate options for offline app

Hi

I was wondering if they are "good" options to manage https with a local server (which is offline from internet).

We tried to use self signed certificate but chrome/ie but navigator shows some errors.

We use a web app, so it can be nice if we haven't anything to install on for each user's computer.

Thinking about using things like letsencrypt but the local server our of internet seems to be a problem too.

Did you see any option for our problem ?

Thank's !