Openwrt router allegedly affects IPv6 connectivity of its neighbour

I have two devices, my computer and an Openwrt-flashed router, both connected to the switch and obtain ipv6 address from it. The router is normal, but after a restart for the computer, after a period the ipv6 connectivity is lost: one cannot ping it from outside, and it can't pass any test on But, the router and the computer can ping each other. Restarting networking and network-manager might restore the situation, but not definitely.

I also asked one of my friends, and he said he had encountered the problem before, and he suggested the router is to blame.
Any advice?