Belkin RT3200/Linksys E8450 WiFi AX discussion

Dear sir, is it possible for you to write a more detailed mesh walkthrough for the web-based interface? I can't seem to handle command lines here. I am thinking of buying another E8450 to be the child node, but I guess the walkthrough at Linksys Official Support - Adding a child node to the Linksys Dual-Band EasyMesh WiFi Router is not applicable to openWRT'd parent node? Do I have to openwrt the child node first? Thanks.