@Marco16 

Why static route? route is automatic added when remote lan is added in vpn configuration.

but what I have seen is that there can be problems if you select several local networks on the tp-link vpn configuration, I myself have this problem against the cisco firewall. Do you need several local LANs in VPN, create several identical VPN tunnels with the same encryption and preshred key.

  @MR.S 

In my Routing table there isn't any route to Site B.

I have 3 route:

No Route to Site B.

And I have only one Subnet in /16.

Site A 172.16.0.0/16

Site B 172.21.0.0/16

  @Marco16 

you don't see s2s vpn routing, can you show a screenshot of your vpn configuration

  @MR.S 

  @Marco16 

it looks right, I don't know fortigate so I can't help there, but if I had a tp-link cisco or unifi router at the other end, this would have worked.

you should not create a static route on TP-Link, it creates this itself based on the vpn configuration

  @Marco16