Please try these options below:
1. Check if your router is up to date with latest firmware update.
2. Make sure your fortigate really has static public IP.
3. Try checking your configuration again if you haven't missed anything, including phases: https://wiki.teltonika-networks.com/view/IPsec_configuration_examples
4. Check if your fortigate firewalls firmware is updated too.
5. Reboot your router after configuration, sometimes IPSec doesn't go up instantly by itself after fresh configuration.
6. Reset your router to factory defaults without keeping any settings or scripts behind.