Not all SFP modules are supported by the Meraki switches, have a read of this article for more information - https://documentation.meraki.com/MS/Other_Topics/MS_Switch_SFP_and_SFP__Overview.
My recommendation would be to first ensure that the SFP is supported and is being recognized by the MS switch.
Get it connect to the Fortigate via an ethernet connection and get it cloud communication. Insert the SFP and see if it's recognized on the dashboard. If not, get in contact with Meraki support and they can have a look at the device for you to see if it's being recognized.
Eliot F | Simplifying IT with Cloud Solutions
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