How to factory reset iMac 2019 Mojave

If the Command-R really doesn't work, the recovery partition might be damaged or missing.

If that is the case, I'd download the Mojave installer (and hope I get the ~6GB full installer, not the tiny stub installer) and make an USB startup disk from that.

https://support.apple.com/en-us/HT201372

Or use the Internet Recovery option by booting via Option-Command-R.

try Alt+CMD+R

If you are saying you can't boot into Recovery Mode, try plugging the charging cable into your keyboard first. Release the command + R keys when you see the Apple logo appear.