Is it safe to install and use app from ”untrusted developer“?
Generally speaking, no, it is not safe as the pop-up states. As you don't know where the app comes from and it has not been checked by Apple's reviewers.

You'd need to install the app developers' certificate to install this app. That certificate could be misused to extract data/control phone/misuse information. Companies often use their own certificates via mobile device management to do such things. Like wiping a stolen phone or blocking the user after leaving the company.

Further Explained

The warning is generated to protect users to install apps from untrusted (not validated, unknown) developers. This mostly occurs on apps being downloaded outside of given app-stores.

In this case (I don't know the app) you really want to lookup the developers site and references.

Maybe it's an open source project. Open Source projects produce this warning but in many cases this warning can be ignored as the project has been reviewed by a bunch of users stating it is safe to install. Still: Caution!.

A search shows a company with the same name is already in the iOS App Store. Though, I don't know whether it's the same company. And there seems to be a reason why the app is not listed in the app store.