Strange behavior with ZTE MF821D (and possible other MDM9200 devices)

[Kristian Evensen]

Hi all,

After spending more time experimenting and trying to solve this
problem, we have sort-of come to a conclusion. In case anyone else
finds it interesting, I thought I should share it here.

Our findings can be summarized as follows:

- The problem is triggered by the switch from 3G to 2G. I can reliably
trigger it by sending AT+ZSNT=1,0,0 to the modem or setting system
selection to 0x0004 (both locking MF821D to 2G).
- The root cause is the modem, but I can trigger the error on
different USB hosts. I have been able to replicate it on the earlier
mention router, my two laptops (with Intel-chipsets), a Raspberry Pi
and some commercial mobile broadband routers. However, some hosts seem
more resilliant than others. One for example the TP-Link I can trigger
it all the time, while on my laptop it is more seldom.
- One theory that we have not been able to verify is that the problem
is somehow related to ISP. I got some friends in another country to
try the AT-command with MF821D and the TP-Link WDR4300, and they did
not see it.
- I tried the same with another modem I have, the Huawei E392. I had
no problems locking to 2G or moving between 3G/2G.
- Our current work-around is to disable 2G. After doing this we have
not seen this problem.

-Kristian