Set an other IPV6 Gateway for Router ADV

Hello,  i would like to set an other defaultgateway in router ADV. Here the config from the interface

config ipv6
    set ip6-allowaccess ping https ssh snmp http capwap
    set ip6-address 2001:470:2e0a:529::1/64
    set ip6-send-adv enable
        config ip6-prefix-list
            edit 2001:470:2e0a:529::/64
                set autonomous-flag enable
                set onlink-flag enable
                set preferred-life-time 309600
            next
        end
end

This time we swiched internal to an Layer3 Swtich, this switch is the new gateway. But ADV from fortigate give as gateway the own interfaceaddress. How can i change the gateway that is given by ADV. Thanks

FortiOS v5.2.6,build711 (GA)