By default Marlin only recognizes files that have extensions that start with G, like .g or .gcode. If you search around the forum a bit, you should find a thread where Leo provided a code change (told me what line to change, that is) so that Marlin will also recognize files that start with N. This simplified things for me quite a bit. This might be why Marlin is not responding to a file sent from Octoprint.
I have wireless octoprint servers on my 3D printers, but I’m still undecided about putting one on my MPCNC. I like the ability to send a file without having to carry SD cards back and forth, but I don’t have much need to start the machine remotely – there is too much setup that has to happen before I start to cut or burn. I just want to get the file onto the SD card, and then use the LCD (and my pendant) to set the machine up.