If you're still figuring out how to configure the Kerlink to use GPRS/3G, things to consider are: choose your ip_link address in /knet/knetd.xml wisely as the auto connect mechanism depends on this. Also make sure the poly packet forwarder is restarted whenever there is an interface change (i.e. ppp0 comes up/goes down) as it will break your IP config if it isn't.
Read more here: https://ipv6.net/blog/how-to-configure-the-kerlink-iot-gateway-using-the-gprs3g-modem/
Cheers!
Erwin