Why my Yeeloong has disappeared from network?

Doing remote things lately with Yeeloong, I was puzzled why it lost SSH link. Looking at a router side, it gave me exact time when it all happened:

Jun 3 07:56:08 m-gw kernel: rl1: link state changed to UP
Jun 3 09:56:57 m-gw kernel: rl1: link state changed to DOWN

This is funny, I simply forgot to plug power cord. 2 hours non-stop on batteries :-)