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I have a RUT955 with two SIMs configured to failover on ping fail. Each SIM has it's own public static IP address. I need to connect an external service (TCP to specific port) to the router automatically which will use whichever SIM is active.

I think it is pretty clear there needs to be an intermediate device deciding which SIM IP to route traffic to but I am struggling to find a solution. I know the router supports HSRP (Hot Standby Router Protocol) which I think is ideal but it only seems to work with two routers on the LAN side only, not one router on the WAN side? Another option is to initiate a GRE tunnel from the router to the intermediate device and route traffic to the endpoint of this tunnel. Any other ideas?


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Do you want to just route the specific traffic through the mobile interface?

Yes. The traffic shall be routed to the router via one of the SIMs (depending which is up) and then handled by the firewall to be forwarded to the local LAN. The issue is being able to determine which SIM IP to route to which must be automatic and seamless. The device initiating the port connection must not know there are two routes hence why there needs to be an intermediate device (server/host) unless there is something I'm not thinking of.

If the port connection could be initiated the other way round i.e. from the local LAN out to the internet this wouldn't be a problem but unfortunately I don't have a choice.