The WireGuard protocol handler passes the tunlink option to netifd as
the interface to create the host routes to the peer endpoints on
(proto_add_host_dependency). Without it, netifd uses the interface with
the most specific route to the endpoint, which is not always the
intended uplink, e.g. with several WAN interfaces or other VPNs.
LuCI has no field for it, so it can only be set with uci. Add it as
"Underlying interface", next to "No Host Routes".
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Erik Conijn <egc112@msn.com>