CLIで設定が必要です。

# config system interface
# edit port1
# set speed {option}
# end

{option}
auto Automatically adjust speed.
10full 10M full-duplex.
10half 10M half-duplex.
100full 100M full-duplex.
100half 100M half-duplex.
1000full 1000M full-duplex.
1000auto 1000M auto adjust.
10000full 10G full-duplex.
10000auto 10G auto.

■Fortinet社のドキュメントライブラリー
※FortiOS7.4.3
https://docs.fortinet.com/document/fortigate/7.4.3/cli-reference/9620/config-system-interface