Just add the option -a <your_public_ip> to the rip44d command line.
If you run rip44d on the gateway it autodetects its gateway address. Behind
a DNAT it can not, so you must add it manually.
And there is no need to open port 520 on the router. The RIP packets are
IPIP encapsulated so only DNAT of proto 4 to your ampr gateway is needed.
Marius, YO2LOJ