OpenWRT Sierra EM7455 drops connection while driving

[Russ Westrem]

On Dec 17, 2018 at 2:26 PM, <Peter Krause <peter at uringme.com>> wrote:

I have a Sietta EM7455 LTE modem in a recent Lede build: OpenWrt SNAPSHOT
r6550-83e1fce
I'm using a USA Sprint SIM.  I did the AT commands to add the Sprint bands
and put the modem in MBIM mode.
ModemManager is v1.8.0

/etc/config/network has the following block:
config interface 'wwan'
        option _orig_ifname 'wwan0'
        option _orig_bridge 'false'
        option proto 'modemmanager'
        option device '/sys/devices/platform/ehci-platform/usb1/1-1'
        option apn 'r.ispsn'
        option auth 'none'

When ModemManager starts, no problems with connection.  It comes right up,
and mmcli says it's in connect mode.

When I drive, the connection is fine for a while, then drops and I can't
bring it back without rebooting the router.  I tried
"/etc/init.d/modemmanager restart" and it will say it has re-connected, but
I can't ping out.  When I reboot the router, everything comes back
immediately on bootup and I can browse again.

I put the modem in debug mode and drove around until it failed, attached
are the mm.log from ModemManager in debug mode, and the /var/log/messages
from Lede/OpenWRT

mm.log:  https://pastebin.com/GY6k9Qyk
messages: https://pastebin.com/aitKE0Mb


Any clue why it won't work while driving around town?  It seems like it
can't recover from a tower handoff, but I know the Sierra EM7455 is
supposed to be able to handle that.

Thanks
Peter Krause

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You need to add cgdcont=3,"IPV4V6","OTASN"


Sprint seems to need this second apn for tower handoffs.
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