The fix on an isolated machine is to put the plugin there yourself, and there are two ways depending on how much you want to automate.
By hand. Download the plugin archive on a machine that does have internet, move it across, and unpack it into the plugins directory the server is configured to read. Match ownership and permissions to the rest of that directory, then restart. The plugin list should then show it as a normal installed plugin rather than the bundled leftover, and the datasources resolve without touching their configuration.
Via the CLI, pointed at a local file or an internal mirror. The plugin install command accepts a URL rather than only a name, so an internal file server or artifact repository works and gives you something repeatable. This is the better answer if you have more than one server, because the manual route is one you will get wrong on the third machine.
Two things to check while you are in there:
- Version compatibility. You want the plugin build that matches the server version you upgraded to, not the newest one available. This is exactly the situation where the newest is not the right answer.
- Whether the old bundled copy is still lying around. Having both can produce confusing results, and cleaning it up after the real one is in place removes a whole category of future puzzlement.
Longer term, the general lesson for an isolated network: assume anything that used to be bundled can stop being bundled. Mirror the plugins you depend on to somewhere inside your perimeter and install from there as a matter of course, so a packaging decision made upstream is no longer able to take your dashboards down on a Tuesday morning.