Finally, after so much time, i have the solution (with the valuable help of an i.t. colleague).
Go to device manager, uninstall the monitor, scan for changes, install the proposed PnP Generic monitor driver.
Funcion keys F4 and F5 now work.
Then, go to Dell's Drivers & Downloads site and download/install the Dell S2230MX Monitor Driver.
Everything now is working flawlessly :-)