I have also a router running in full remote, nearly 4000 km away. For the reboot you can try to switch of power by your customers, it's more easy for them than a login in the webinterface and start the reboot there. If you are successfull to reconnect the device again, activate the autoreboot, maybe everyday at midnight. Or try the reboot by SMS-command (but this feature must be activated). Godd luck!