OpenWRT Sierra EM7455 drops connection while driving

[Peter Krause]

I have a Sietta EM7455 LTE modem in a recent Lede build: OpenWrt SNAPSHOT r6550-83e1fceI'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/GY6k9Qykmessages: 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.
ThanksPeter Krause
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