Hi Pjmice,
Thanks for using Apple Support Communities. We understand you’re having trouble using your printer after upgrading to macOS Monterey. We’re happy to provide some information.
This support article from the Brother website offers a couple different workarounds for using that printer model with macOS Monterey: Macintosh macOS 12 Monterey - Support- MFC7860DW - Brother
If those do not work, and you’d like to continue using this printer model, you do have the option to go back to the software version your Mac shipped with, and then upgrade to Big Sur from there. You can find steps for doing so under “Other macOS installation options” here: How to reinstall macOS - Apple Support
We also recommend creating a backup before reinstalling macOS, should you choose to do so: How to back up your Mac - Official Apple Support
We hope that helps.
Take care.