Set ipv6 default route

I have a box that sends out radvd announcements on a lan, but something on the lan makes the other machines forget the advertisement about the default gateway after a while. They remember their ipv6 address, but cannot connect outside the lan anymore.

Before I used to log in to the machine with radvd and restart it so that it sends out fresh advertisements, but I'm now trying to explicitly add the default route with

read -p host: m;
ssh -t ${m:?} sudo ip -6 route add default via fe80::2b0:d0ff:febe:5fd7 dev eth0

Let's see how that goes...