1) Login via CLI SSH and try ping to any of the FQDN and make sure it able to resolve.
exe ping www.fortinet.com
2) If not able to resolve, check configured DNS setting, if the system DNS is correct, then check step (3).
show system dns
3) Make sure the outgoing DNS port is allow by your upstream device, if there is DNS response, you should see the response accordingly.
diag sniffer packet any 'port 53' 4 0
4) Also, make sure your upstream device allow outgoing port TCP/443.