QMI and LTE: IP connection problems

Hi all,

I'm testing around with a Sierra Wireless MC7710 in QMI mode. For this I've checked out, built and installed 
the current versions of ModemManager and NetworkManager a few days ago. First tests with two different UMTS 
networks were successful. Great work, this really works out of the box both with QMI and AT commands (Sierra 
Wireless SW/DirectIP mode)!

For further tests I'm using a communication tester for LTE signaling. Currently this fails at the simple IPv4 
connection. I think I can exclude the communication tester as the connection works flawlessly using the 
module with AT commands in the SW/DirectIP mode.
So what is happening in QMI: The module gets attached and gets an IP address with the default bearer. Setting 
up a connection either using the NetworkManager or mmcli --simple-connect works. Binding this IP address to 
wwan0 using dhclient works, too. But while after this procedure with AT commands (SW/DirectIP mode) I can 
ping my APN, use services which the communication tester provides and so on, no IP-based communication is 
possible when having set up the connection via QMI. 

What have I done already: 
- Checked output of ifconfig wwan0. Differences in the netmask: In AT mode the netmask is, in 
QMI it's I don't know yet if this could make the important difference as in general the UE 
only needs to connect to its gateway which can be done with a rather limiting netmask, too.
- Changed configuration of wwan0 to the data which works for the AT mode - no success. 
- Tried pinging the UE from the communication tester using a built-in ping functionality. It got pongs back 
when the UE was attached but stopped as soon as I connected the UE using the NetworkManager or mmcli 
--simple-connect. When I disconnected the pongs re-appeared. I cannot explain this to me in any way...

Does anyone of you know what could be the problem in this setup? Can I provide you with more information or 
log files to narrow the down problem? Which parts of the ModemManager log is crucial?


PS: I tried to gather some experience in a live network but unfortunately our provider seems to be unable to 
offer LTE for a few days already now. 
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