Hello,
Our controllers have a watchdog in case of communication failure. This means that the command should be sent periodically in order to have it work.
Default watchdog value is 1000 ms. So the M command should be sent maximum every 1 second. You can disable it by setting ^rwd 0 (for more details advice the user manual). Do it in order to make the set up, but we do not recommend it in runtime modes.