My home configuration.

Internal LAN 10.10.0.0/24 -> FortiGate -> Internal WAN/LAN 172.16.10.0/24 ->TPLink A9 AC1900-> Public IP space (Xfinity) Cable modem in bridge mode-> Intenet

I want to be able to route the 10.10.0.0.network to the intenet without needing to a rule that requires NAT on the Fortigate.

Static route exists on TPLink for 10.10.0.0/24 sending the WAN interface of Fortigate 172.16.10.2

If I enable NAT on the Fortigate and NAT to the WAN IP of the Fortigate I can ping the world and get responses

If I disable NAT on the Fortigate and do not nat (source IP remains 10.10.0.0 IP I can only ping to the Internal inteface of the TPLink 172.16.10.1

Can TPLink perform NAT for IPs that are not members of its own LAN?   

Been doing networking for a long time...what am I missing?  Is this a limitation of TPLink?