The easiest way I know to do this is to just accept the route it's advertising. Once it's your inet6 router, it's easy.
$ ip -f inet6 route
2001:db8:dead:beef::/64 dev eth1 proto kernel metric 256 expires 3523sec mtu 1500 advmss 1440 hoplimit 0
fe80::/64 dev eth1 proto kernel metric 256 mtu 1500 advmss 1440 hoplimit 0
default via fe80::21b:63ff:fe12:3456 dev eth1 proto kernel metric 1024 expires 1722sec mtu 1500 advmss 1440 hoplimit 64
'default via' is my router here. Let's see who it actually is, by querying my ARP neighbors.
$ ip -f inet6 neigh
fe80::21b:63ff:fe12:3456 dev eth1 lladdr 00:1b:63:12:34:56 router STALE
$ ip -f inet neigh
10.0.0.1 dev eth1 lladdr 00:1b:63:12:34:56 STALE
So I've found an inet6 router with a MAC address 00:1b:63:12:34:56, and a v4 address 10.0.0.1. Hopefully that'd be enough clues to send you back into familiar territory.