Not sure if this problem has been appearing since the last update, the my router is connected to my main router with internet cable, when i try to connect any device to the router by wireless the connection drops after few seconds, any clues of what is happening?.
I have had this happen and it was a faulty USB power supply. When connecting on wifi the power supply voltage was dropping to 3volts or less and the mt300m was crashing or locking up or just dropping the wifi only for it to reappear.
I borrowed my phone charger to test and all was fine.
I don’t know if this is your problem but worth a try 
It was the power supply!, tested with my smartphone power supply and it is working again. I have been having a hard time trying to fix this just because a cheap power supply.
Thanks!