Hi, I still have the problem on two MR6400 (in the maintime I bought another ...) and I discovered another user has the same problem.

Since I first thought it was a problem of ISP, cooperating with them, I can exclude problem on their side. I've, contrarily, focused precisely the situation the generates the problem: it happens just when to modem are connected smartphone (Andorid) or Linux PC - not happening when Windows PC is connected. Infact, when one Linux/Android device is connected and, due to reading a long doc/page, the communication does not generate data flow, the model "stalls" and sometimes return alive in 30-60sec, sometime it does not get back.
Windows is always generating data flow (150bps minimum) due to continuos calls to Microsoft server, that keep communication alive.

WHAT CAN I DO ???

https://community.tp-link.com/en/home/forum/topic/532776
https://community.tp-link.com/en/home/forum/topic/532776?replyId=1048916
https://community.tp-link.com/en/home/uc/post/938586