@RJanardh Does the speed from connecting to the base unit work better than the satellite?  And is the wired speed still ~400mbps?  Lastly did you try to re-run the QOS speed test and reset the maximums? The U/D speed in the QOS screen is an effective cap on all devices.

From reading the other posts about M5 performance (I am having an issue, but it's not quite what you are describing) the one thing that helps with unexpected drops in Wi-Fi performance is a factory reset of the system. 72mbps is the slowest negotiated speed for AC wifi, so its almost like the units are 'stuck' in slow mode and wont return to faster speeds automatically as they should, except for a hard reset.