Note that the article that you have there would not only specify to run
FASYL-FIREWALL # diagnose debug flow filter addr 10.10.1.1
This will only set a filter. What also needs to be done is diag debug enable and starting the trace.
To copy from that article with your IP:
diagnose debug enable
diagnose debug flow filter addr 10.10.1.1
diagnose debug flow show function-name enable
diagnose debug flow show iprope enable (this is good for routing decisions)
diagnose debug flow trace start 100
Also, the sniffer command with that IP:
diag sniffer packet any 'host 10.10.1.1' 4 0 a
would and should give output. If there is too much output, replace the 10.10.1.1 with your client host IP.
Troubleshoot with understanding what is troubleshooted - this here attempts to show what happens with a packet when it arrives at FortiGate, how it is processed and sent elsewhere:
https://docs.fortinet.com/document/fortigate/6.4.0/parallel-path-processing-life-of-a-packet/86811/packet-flow-ingress-and-egress-fortigates-without-network-processor-offloading
The sniffer command would start at the network interface.
The flow trace would be somewhere at the "kernel" part.
flow trace will show inbound packets, if they arrive, and say what happens to them, policy and routing decision.
If the flow trace shows output, share it, if possible.
If the flow trace shows no output, run the sniffer.
If the sniffer shows output, share it, if possible.
If the sniffer shows no output, your FortiGate does not receive packets from your host.
Then I'd say check the routing path from your client to the FortiGate.
The sniffer and flow trace may also show that packets arrive, but the response isn't sent or sent on the wrong interface. In that case FortiGate routing may need adjustment (config router static)