You don't have to add the static routing yourself. The "route add" command on the backup server is done automatically. It's only if you want to reach a test VM from another machine in the same subnet as the backup server, that you can add it to reach those "masked vms". Why in the same network? You core router doesn't understand the Veeam masquerading layer so it doesn't know how to forward it to the router.
However if you want to map an external ip to an internal ip aka static ip mapping (are you talking about this https://helpcenter.veeam.com/docs/backu ... tml?ver=95
?) then the IP you want to use should be on the same subnet as the backup server and the appliance. This because packages will be intercepted on the same interface as the production network that you defined here
. In this case, that IP can also be reached by a machine that has not the "route added" because the appliance will NAT a production ip to the internal ip