even issuing the command SET MODE-CFG ENABLE the SET DOMAIN is not available. Does this command is only available for remote IPSEC VPN or is available for SITE TO SITE IPSEC vpn? I cant make it work my DNS resolution..
The respective configuration only works for Dynamic VPN tunnel. If you are having a Site-to-Site VPN configuration, it is not possible to set the domain. From where do you want to ping the remote site server? If it is FortiGate, you can change the DNS setting in FortiGate, if you are trying to ping from the internal hosts, you can check if it is configured with an internal DNS server that can resolves the server name on the remote network.