I also have the impression it is the firmware version, since this is the only thing which appears to have changed during recent weeks on my MT3000. Though I even have no idea how to downgrade properly. And I have subjectively no time to play around with this.
I will now give a judgement on this product: it cost around €100 and alone in this thread, I have invested more time already than this server is worth.
I come to the conclusion that this product as a whole has been a HUGE DISAPPOINTMENT so far. I'll try to upgrade to 4.7 once this is available. Why does still no-one analyze my logs, there should be a clue in there what goes wrong here. Interestingly the NGINX error log is empty, and there is no error pointed out explicitly in the logs, nor is the device giving any clear signal that there is no WLAN signal (just some blue light).
Now if what I describe here is no error ... . I would not recommend any prospective customer losing time with this product, despite buying it, based on the experience so far. I have a Vodafone station from the ISP which had no such downtime since various years, and it is just a very basic router (not even a Fritzbox). Guess I should go for something like a Fritzbox instead.