Once an iOS firmware update has been out for long enough to verify there are no critical issues (usually about 30 days), Apple stop signing the prior version. Once they do that, you can’t roll back.
So unless it was a very recent (last 30 days) update to a just-released firmware, you’re probably stuck on the version you have.
It may simply be that you need to update the firmware on the Lightning to USB 3 Camera Adapter.
To do that, restart the iPod with nothing connected to it. Make sure it is connected to WiFi and can reach the internet. Then plug the adapter into the iPod without anything else connected to the adapter.
If a firmware update is available, you should be prompted to install it. Might take thirty seconds to pop up after you connect the adapter. And you can check manually via Settings | General and look at the attached adapter’s information there.
Note, depending on what version of iOS you’re current running on the iPod you may have to bring that as current as possible first - particularly if it’s a version that predates the accessory update capability.