You cannot merge all the link to get the combined speed, but instead make use of SDWAN with SDWAN rules to steer traffic to different WAN links based on criteria's and also apply SLA to monitor each link for Latency Jitter & Packet Loss etc, if one of the link fails to meet the SLA thresholds SDWAN can shift the traffic to next available link based on the configuration.
There is no merging of link but you can load balance between them .
FortiGate SD-WAN allows for session-based load balancing. This means it will distribute new sessions across the WAN links based on your configuration (like weights, least latency, lowest packet loss, etc.).
Ensure you properly configure load balancing settings based on your needs.
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.