Mange fortiswitch over ipsec tunnel

good to all,
I find myself in this situation: 

The goal is to allow the management of the FortiSwitch directly from the GUI of the FortiGate 91G located on site, even if the firewall and the switch are not directly connected via the FortiLink port. However, as shown in the diagram, there is an IPsec VPN tunnel.
As per the attached diagram, the configuration already presents the IPSec VPN tunnls with related policies and static routes.