Out of range signal (Monitor problem)

FWIW. I had this issue today when I performed a restart of my 2018 Mac mini. In my case I too have a Dell monitor but it is a 4K model. Unplugging and replugging in the HDMI cable resolved this as well. It doesn't happen on the first boot up or subsequent restarts, but only during a random restart. I have a HDMI cable directly connected between the mini and the monitor.

When running macOS Catalina 10.15, I was not able to get a display on the monitor using a direct HDMI connection. I had to use a USB-C-to-HDMI adapter for this to work consistently. With the release of 10.15.1, I was able to go back to just using the HDMI connection directly ... but did come across this exact issue once ... so 10.15.1 sorta fixed it, but looks like it will require a 10.15.2 to get this sorted out.

Thanks, but I don't think I will be trading in my 2018 model for a 2012 one anytime soon. Of course, I'm assuming that by "same monitor" you are talking about the OP's FHD model? For clarification, as I mentioned in my reply, I have a Dell 4K monitor. It's the model: U2718Q.

No doubt there is something amiss with the 2018 mini that is causing these issues. So far an upgrade of software (99% fixed it) for me ... but this may be a firmware or hardware issue that will still need to be addressed. I still suggest that you try using a USB-C-to-HDMI adapter in the mean time to see if it will resolve the problem are you are seeing with yours. Also be sure that you are using a good quality HDMI cable. These cables are actually rated and you may want to obtain one that is one higher rated than what your monitor can actually display. FYI, the lowest-rated HDMI cable is marked as "Standard." The next level up is marked as "High Speed." Finally, there is those marked as "Ultra High Speed," but these would only be required for 5K, 8K, or 10K resolution capable monitors.

Go to the Apple store or borrow a HDMI monitor from a neighbour/colleague - it'll work. Your monitor is obsolete, meaning it is out of date and unsupported. You could wait for a Catalina fix or try assorted dongles but I wouldn't recommend it. Good luck.

Reset the NVRAM, as per > Reset NVRAM or PRAM on your Mac - Apple Support

Then if that doesn't help, > Use safe mode to isolate issues with your Mac - Apple Support

Let us know if the adapter works for you. This is the one I used: https://www.apple.com/shop/product/HKQ22ZM/A/belkin-usb-c-to-hdmi-adapter?fnode=8b&fs=f%3Dadapter%26fh%3D4595%252B45b0

Go to the Dell Menu and see if selecting a different input source or aspect ratio helps.

OP’s monitor doesn’t have HDMI, only DVI, VGA, Composite and S-Video.

Chuck the monitor and get a new one, it’s obsolete.