configure site to site or any other vpn without public ip between two devices on firtigate

You can configure Dynamic DNS and use FQDN in-place if IP address.

https://docs.fortinet.com/document/fortigate/6.0.0/handbook/771978/dynamic-dns-configuration

Hi Omarlodhi,

Yes you can configure the site to site VPN between devices without public IP address on Fortigate using hostname or DDNS name; however both devices should be able to communicate to each other on internet. 

On Fortigate device to configure the VPN using DDNS, please check below snapshot:

Regards,

Parteek