As the router could not drill the 4mm holes in the steel plate I used a cordless drill for a few holes.
I recognized that my cordless drill was getting warm and that means the motor is working hard. An overheated motor reduces the lifetime of an electric tool drastically.
I did not want to damage my cordless drill and looked for other options with the tools I had available.
The perfect tool would be a magnetic drill that is held down to the steel plate by the magnetic feet.
But I don´t have such a tool and renting one is quite expensive.
So I went with a poor man´s choice and used some clamps to fix my drill press to the steel plate and started to drill the holes.
That went surprisingly well and quick, even I had to move the drill press and clamp it again for every hole.
I managed to drill half of the 96 holes before I had to stop. Yes, there is just a little time between the end of work and the time you don´t want to bother your neighbors with the noise of your drill press.

My holes are 50mm apart from each other and the idea is that those M5 threaded holes are used to bolt down other fixture plates like wood, aluminum and whatever can be milled flat by the router and in some cases could be sacrificed during the milling of parts.

This is the setup I used for the attempt to drill M4 holes with the router. I did not work, but I learned a  lot about the Fusion 360 CAM module again, especially about the drill operations.

I can only strongly recommend using the simulation function before sending any G code to your CNC. It will show you exactly what the CNC will do. It is much easier to fix any issues on the computer than on a pile of ruined parts.