Configuration of new FortiSwitch Standalone

Hello Community, i'm new to the world of FortiSwitch (i had configure a lot of FortiGate) but i'm in trouble with the switch.

So, we have for a customer a FortiSwitch that i must configure.

The customer have one switch L3 (Aruba, not FortiSwitch) with all the ports untagged on VLan 1 default and tagged for all the others Vlan's (Vlan10, Vlan3, Vlan20). The L3 switch have an IP Address on every Vlan's (On VLan10 192.168.0.1, On VLan3 192.168.3.1 and on Vlan20 192.168.20.1), and is direclty connect with ALL the others switch

All the switch's of customer have an IP on 192.168.3.0/24 that is the MGMT VLAN (Vlan3)

So, i want to add in a floor, the FortiSwitch 124F PoE, and i have problems with the configuration:

- I changed the Physical address for 0.0.0.0/0 to 192.168.3.6 (the Ip choice). I left the Vlan ID1 with the default IP 192.168.1.99

- I added a static route 0.0.0.0/0 to 192.168.3.1 (The L3 Switch) 

- I changed the Port 25 SFP with native VLAN ID1 and Allowed all the others VLAN's and i left the internal configuration as it was (VLAN Native 1 and allowed the others VLAN's)

And..nothing to do.

I try to left the IP address of Physical interface to 0.0.0.0 and add the VLAN3 under System-Network-Interface adding a seocndary IP address 192.168.3.6 and ID Vlan 3 but nothing to do, i cannot ping the switch. 

What i wrong?