This a bit dumb question but isn't it so you really cannot send new line by typing \n or something else into serial monitor.
My code recognizes other characters so my code has not that problem but I thought there were ways to send newline. Other than that option at lower edge of the window.
Too bad there is no serial.readln command.
The Arduino serial monitor is quite limited.
Try one of the free serial monitors on the internet.
I like Realterm.
Yes, I have Realterm already and it works. I was just a little stubborn and wanted to try my way first. And of course I plan to use another microcontroller to send commands to this via serial, so it does not really matter what the serial monitor is able to do.
Any reason why you don't set line-ending to newline? Text will be send including a '\n' at the moment that you hit send.