My NB-IoT shield deluxe occationally looses connection with the T-mobile network


My NB-IoT board de Luxe occasionally seems to loose its connection with the T-mobile network. It’s the board we got during the training on Nov 3rd, so with a SARA N211 and the new 6.57 firmware. Derived from the demo sketch we used during the training I wrote a sketch that repeats sending data every 30 seconds (AT+NMGS=25,“0bb6107503ed004eab330008b1e348454c4c4f2046454c4958”).

What I observe when the connection is lost, is: “AT+CGATT?” returns a “0” and the “AT+CSQ” returns signal strength “99.99”.

Sometimes the connection can be restored by issuing "AT+COPS=1,2,”20416” and wait until connection is restored ("AT+CGATT?” return a 1). Sometimes I even have to switch off/on the radio.

Does this sound familiar to someone, and how to solve it in a robust manor?

Regards, Felix

With the same board, when using the Sodaq_NB-IoT_All_Sensors sketch, connectivity is lost after ~1 to 2 hours if the board is stationary, and more often (~every 15 minutes) when the board is mobile (walking speed). A “[rdResp]: REBOOT_CAUSE_APPLICATION_WATCHDOG” is logged, thereafter all rdResp are ERROR, until the board is restarted.
Is it known what the root cause is, and how this can best be fixed?

Hi Guys,

The example sketch isn’t perfect.

It’s known that when you move the board needs to reconnect to a new cell.
This should be done automatically. but for a short period of time you are not connected.

With the new firmware the sketch will go into sleep.
The command "AT+CGATT?” "AT+CGATT=1”should wake the module. and after a few seconds you should be reconnected.

Please keep sending us your feedback.
This way you help us improve the library.
We are working a new “official” version.

Best regards,

From contact I had with T-mobile I learned that there might be an incompatibility between the new uBlox G6.57 firmware and the current T-mobile network software. The network software is expected to be updated by the end of this month and the problem should then be solved (to be confirmed of course). In the meantime I’ve added a work around to my sketch: before sending new data to the network, test if the modem is still connected to the network. If not, re-establish the connection by:

  • switch Radio off
  • set IP of Chip and Set Vendor CDP again
  • set PDP and ANP again
  • switch Radio on again
  • register With Network Operator again

It seems to work for me (at least for 10 hours now).


P.s. the old board with N200 and the new board with N211 but with old firmware don’t seem to have this problem.


Hi Jan, is this still te best method for reconnecting?
And do you need both commands after each other, or is one of both sufficient?

Doesn’t the module reconnect when a AT+NMGS command is send? Or does it not respond to that when its in sleep mode?


Dear Hans,

The library should handle this now.
If you have upgraded to the latest Sara Firmware: FW 06.57-A07.03

FYI, T-Mobile did change their backend:

Kind regards,