There is a Mac Startup Key Combination that should accomplish what you want directly. Use the following:
This will reinstall the macOS that came with your Mac, or the closest version still available, which should be macOS 10.14 for the Mac mini 2018. In order for this to work, you need an internet connection.
Apple has the full directions for starting up in Recovery Mode at https://support.apple.com/en-us/HT204904
The other way to install macOS 2014 would be to create a USB boot installer, as discussed by Apple at https://support.apple.com/en-us/HT201372.
One of these methods should allow you to install Mojave regardless of the version of your Recovery volume.