[tor-relays] Relay behind VPN router

I have a TOR relay behind a Tomato router on an ADSL line. I activated the vpn client to route all traffic through privateinternetaccess.com.

Because a TOR relay does not work behind NAT without port forwarding, I'm looking for a way how to route tor traffic directly to my internet provider. I can add static routes, but I don't now the IPs in advance. Maybe there is no problem to route outgoing traffic through the vpn tunnel, but how can I make my relay publish my WAN IP instead of the tunnel IP? My WAN IP is changing every day, so I cannot configure it as static.