I have two TELEFONIKA RUTX09 and we use those as SMS Gateways.
First Gateway is configured with IP 192.168.10.10.
How can I configure the second sms gateway in a way that if the first gateway isnt reachable any longer (crash or offline), the second gateway operates with the IP Address of the first gateway on WAN side then?
So the IP 192.168.10.10 stays the same and wont change to 192.168.10.11 or something else.
In your case, it seems that VRRP can be useful. With VRRP, both devices can share a single virtual IP address. One RUTX09 would be a Master device which would be used if its up, and the second RUTX09 would be a backup. If the Master RUTX09 becomes unavailable, the backup RUTX09 will become active and use the same virtual IP address. We have a configuration example for VRRP on our wiki here.