Thanks for your answer.
As this issue happens on very few occasions, I will hardly be able to get the result of these commands when it’s working bad. However, I have found what might be the cause of this issue : each time the issue occurred, the date was set to “Sun Jan 6 1980”. I think it makes sense that the network access is denied when the date is wrong.
Now the question is, what could have caused the reset of the date ? Maybe an issue with the GNSS (which may also provide time) ?
Here is the ouput in “good” state (in case it can be useful) :
root@swi-mdm9x15:~# cm radio
Current Network Operator: Orange F
RAT: UMTS network (LE_MRC_RAT_UMTS)
Status: Registered to a roaming network (LE_MRC_REG_ROAMING)
Signal: Strong signal strength (4)
root@swi-mdm9x15:~# cm sim
SIM card is inserted and unlocked (LE_SIM_READY).
root@swi-mdm9x15:~# cm data
PDP Type: IPV4V6
root@swi-mdm9x15:~# microcom -E /dev/ttyAT
00, All bands
!SELRAT: 00, Automatic