I found this article:
I think it is a FortiOS 7.0 feature since QinQ is not mentioned in any
of the 6.4 administration guides that I've read through.
Why is nattraversal forced? Let it automatically figure out if NAT is
needed or not, or better yet disable it if you know both sites are
directly connected to the internet via public IP-addresses. Also, what's
the latency between your sites? Latency ...
1500 byte is the default and recommended MTU. If you start increasing
the MTU then you need to make sure the full path supports this increased
MTU, otherwise you will have issues.Recommended MSS is MTU - 40 for
ipv4, so with a 1500-byte MTU you want ...