Quectel EP06 modem "transaction timed out" lockup

[Alex Ballmer]

I have a series of Quectel EP06 modems that occasionally experience a 
deadlock condition. I am running openwrt 18.06.8 using modemmanager 
1.14.0 and libqmi 1.26.2.


After being online for some time, the modems will eventually lose the 
ability to ping. mmlci still shows the same bearer as when it was 
working. Netifd does not show wwan0 (the network interface for that 
modem) as having an address.

Attempting to reset the modem via mmcli returns

error: couldn't reset the modem: 
'GDBus.Error:org.freedesktop.libqmi.Error.Core.Timeout: Transaction 
timed out'


Resetting via AT commands (echo -ne "AT+CFUN=1,1\r\n" > /dev/ttyUSB3) 
does not result in any response from the modem. Rebooting the affected 
machine causes the modem to function normally again.

It should be noted that when the modem enters this state, mmcli still 
reports it as being connected.

Is this an issue with the modem itself or something that can be worked 
around in software?

Thanks