You got me at connecetion... for a 3g dongle there usually AT command to do that... maybe for 4g is is the same.
If dialing/connection parameters are properly configured you could try:
if down [4g-iface]
if up [4g-iface]
if not, you could try to unbind USB port (depending on hardware usually acts as if you unplug and replug again)
If I remember correctly:
root@OpenWrt:~# echo "usb1"> /sys/bus/usb/drivers/usb/unbind
root@OpenWrt:~# echo "usb1"> /sys/bus/usb/drivers/usb/bind
or:
opkg install usbreset
usbreset # here you should put your vid:pid or usb kernel path
you could try to reset or rebind between if down - if up ... try a "connection keep alive" script, a continuos ping or something.
If you run reboot the connections comes up again? you could add a watchdog to reboot automatically a minute after connection lost.