@themonet Looks like you used M552 SO, with an upper case 'o' not '0' ie zero.
See this thread about someone having issues with SerialTools: https://forum.duet3d.com/topic/23545/mini5-wifi-set-up
SerialTools is a bit finicky about input; you have to get the command right, without pressing delete or cursor keys. I think it sends each character to the Duet as you type them, rather than when you press return. If you navigate away from SerialTools to another application, and then back, in the middle of writing the command, it won't work. You should get an 'ok' when a command goes through correctly.
Also, make sure you're using straight quotes, eg " rather than curly quotes, eg ”
Ian