I'd been using the Shield DVR server for years, but with it being unsupported I moved the DVR to an old laptop. It's working OK but doesn't support upgrade to Windows 11. I found this tool that changes Windows to an IoT version that gets updates until 2032 (a somewhat limited version but OK for a DVR PC). It took a couple of hours to install on my anemic PC, but it worked and the DVR was still installed and working.
https://teknixstuff.com/Libraries/Projects/UpDownTool/
I'll update this if I run into any issues. It's not an official Microsoft build so don't expect any support other than updates.
Note: I found out about the tool here: I'm sticking with Windows 10 even after it dies next month. Here's how | PCWorld which also includes ways to extend official support.