What is the best way to update/register a Fortigate 40F behind another Fortigate?

[Floto]

Good morning,

I have a question about what is the best way to update/register a Fortigate 40F behind another Fortigate? There is a big rollout coming up, we want to replace all of our Fortigate 40C with Fortigate 40F and I would like to send the fortigates up to date and registered.

We have a lot of small branches and usually we configure new Fortis at our main location just to get them online after they arrive at one branch. Then we register and update the Fortigate, this obviously causes an internet loss.

I’m kinda shure that there is a better way, but I can’t figure out how to do it. First I tried to update the firmware manually. But after one successful update I receive this Error-Message:

This is a FortiOS v7.6.0-build3401 firmware image that cannot be installed because the device's FortiGuard license for firmware upgrades could not be verified or may have expired. Verify or renew the license to install upgrades.

If I understand that correctly the Fortigate needs internet access. So I set the WAN interface to an IP in the same subnet where our main-fortigate is. There is a rule on our main-Forti for internet access, were I added the new Forti. Unfortunately it didn't work, the new Forti is not online.

Then I tried to add the new Forti to our FortiManager. Our FortiManager is in a different subnet, but has access to the other one. In the WAN interface of the new Forti FMG-Acces is checked. Unfortunately that didn't work either.

What is the better/easier way? Get the Fortigate online or add it to the FortiManager? And how I do it?

Hope someone can help me, as you may have already guessed, I'm not the greatest fortigate expert.

With best regards from Germany,
Florian