I think I found the problem.. maybe?
Both routers are connected to the same ISP-switch.
They have their own WAN IP.. but the WAN GW are the same.
I've setup a RUTX11 with 4G Connections and was able to setup IPsec Tunnel to one of the RUT300.
Then I can ping between devices, so I will move one of the RUT300 to another location and test the Tunnel - hopefully it will work :)