Mount a previously internal and encrypted HDD as an external drive I upgraded the hard drive on my late 2015 iMac from the fusion drive that came with it to a 1TB Samsung SSD. My SSD has a fresh install of macOS and I planned on using my fusion drive as an external drive with a SATA to USB case, so I didn’t copy any of my files from the fusion drive expecting to be able to just plug it into the iMac later.

But it doesn’t get recognized at all, not even in disk utility. I’m assuming it’s due to the fusion drive being encrypted with the macOS that was installed on the fusion drive itself. Is there any way I can mount that drive to access my data? Or would I have to open up the iMac again and reinstall the fusion drive, copy my files properly, then reinstall the SSD?
You seem to be under the impression that the hard drive is the Fusion Drive - that is not the case. The Fusion Drive is a "virtual drive" consisting of both the hard drive as well as the internal SSD that was factory installed in your iMac.

You cannot remove just the hard drive from the iMac and expect to be able to mount it externally on other computers and copy files off it. You will need the data from the internal SSD to be able to do that. The easiest way to do this is to mount it in the same iMac that it came from.

If you during reinstallation of macOS cleared the internal SSD of the iMac, you have effectively wiped your data - and you won't be able to mount the external hard drive. You will need to do data recovery on the drive to be able to get anything out.

If you haven't cleared the internal SSD, it should be possible to mount the Fusion Drive again. It is however usually very cumbersome to do so if you have altered the disk configuration by adding another internal SSD. If you use any of the standard tools for fusing the Fusion Drive (like "diskutil resetFusion" on Mojave), you'll not only loose your data - but it will probably just error out stating that you have a non-standard disk configuration.

I would advise simply removing the new SSD from the iMac, putting back the hard drive. Cross your fingers and hope everything boots up - and then copy off any data you want to save. Then you install the new SSD again.