we appreciate your inquiry.
Is SNMP enabled on the fortigate?
You can confirm this by downloading our latest SNMP Tester, run it on the PRTG Host (or host of the Remote Probe), and perform the Read Device Uptime test against the target device, please ensure to configure the proper community, port, device ip/name and SNMP Version. Your result should be an uptime reading above 0.
If the test above is successful this means that SNMP is enabled, in that case please use the SNMP Tester again but this time perform the "Walk" test and enter the following OID as starting point for the walk: 220.127.116.11.4.1.12318.104.22.168
The walk above (if it succeeds) should display the current values and OID's (with index, if there is one) and you will be able to copy and paste the OID's into PRTG which should allow you to monitor those OID's within PRTG.