May I know if you have downloaded the Forticlient from your support portal? Please check by downloading another version of Forticlient from your support portal and try it for iphone users to see if that works.
Is it possible that those iphone users are using only IPv6 in their network? -> Did they get a local IPv4 IP before connecting to the SSLVPN? -> Perhaps from FGT side we can see some logs by filtering the SSLVPN IP that they are getting
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.