The DIR859 has a weak Qualcomm Atheros QCA9563 SoC. Expecting 400 mbps routed performance might be optimistic with OpenWrt.
https://forum.openwrt.org/t/so-you-have-500mbps-1gbps-fiber-and-need-a-router-read-this-first/90305
What version of OpenWrt is installed?
Is packet steering and software flow offloading enabled?