Do not rejoice too early, as IPsec VPN traffic protocols are not so difficult to detect, and to block.
As a hint, since FortiOS v7.0, you can change the default VPN ports (and 4500) to _one_ custom port. The other, receiving site just has to know and be able to configure that too.
OTOH, SSL VPN can be set up to use the standard HTTPS port 443 which is much more difficult to block as that would block a lot of legitimate traffic. Since FortiOS v7.0, you can configure a FGT to act as an SSL VPN client (just like a FortiClient), so you can effectively create an SSL VPN site-to-site.
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