Answer: “yes” to all of questions.
I will say more. Our other device at the same location (with a local SIM-card), but with only one difference in the WS6318 version of firmware to work properly. Working firmware version is R32, non-operating - L30.
The only way I can explain this behavior is the effect of EMI. Perhaps African network operates in other frequency bands and other timing relationships. The modem is restarted even before the first AT commands. We will try to update the modem firmware in place at the client.