We use DUO via Radius with our Fortigates. You can push, call and passcode via the app. You have to have a Duo proxy as well to authenticate as well. Just follow the instructions from Duo and it will be great. We have been using it for 5 years now with no problems.
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.