Backup Link with FortiExtender 511F

[FortiNoob3]

Hello,

We have a remote office with the following equipment:
1)Fortigate 40F

2)FortiExtender 511f

We want to connect to the main office with site-to-site vpn and the Fortiextender to be used as backup wan link. 

Is it better to connect first FortiExtender with the ISP dsl modem and behind Fortiextender the FGT 40F?? 

(ISP DSL Modem-->FortiExtender-->FGT 40F-->LAN ??)

Could somebody explain how to implement this?? 
I'm really confused

Thank you in advance.

[gfleming]

Use the FortiExtender as a Wireless WAN connection for your FortiGate and keep your primary ISP on your FortiGate. This way your FortiGate can leverage features like SD-WAN etc.

The FortiExtender will become part of the FortiGate and present a new Interface for LTE/5G connectivity. It's really easy to set up.

here's some docs:

https://docs.fortinet.com/document/fortiextender/7.2.4/admin-guide-fgt-managed/103815/fortiextender-...