Thank you for question. If you are expanding subnet, I would check couple of things:
- Verify that the new subnet has correct address in all firewall policies
- If FortiGate is also DHCP server, adjust settings there too.
- Verify if there is no subnet overlap and check also overlap with VIPs and ippools
Start with simple ping to internet, check debug flow and packet capture.
Check if traffic is allowed or being blocked, based on that you can follow up with troubleshooting: