First let me suggest that you should have all the hardware on the same subnet so you will be able to see the other devices. The approach as listed has you "double Nat"ting which may lead to issues with some services.
Next, I am wondering if there is something incorrect with the IP assignments you have for the modem and the WD N900. Normally the modem "assigns" the IP to the router, so I would expect that the WD N900 gets an IP in some other IP range (not 192.168.x.x, which is a reserved private IP range)on the WAN side. I could be wrong, not sure what your ISP does.
I will assume all devices other than the modem are on OpenWrt or Lede. Please confirm and if so what versions. Worth stating the model of the Netgear router. You also appear to not be offering wireless on the first floor.
In any event I will suggest you read up on the "dumb AP" and configure all your devices behind the "Gateway" (which ever device that turns out to be) in this fashion.
https://wiki.openwrt.org/doc/recipes/dumbap
So assuming your modem is truly the Gateway, then:
WD Mynet N900 192.168.1.2
PI3 192.168.1.3
Netgear Router 192.168.1.4
You want to set the fixed IP on the above devices or you will not be able to access them if the primary Gateway is off line.
I would get the WD and Netgear up and running first with all the clients and make sure all works.
The Rpi is another story and not positive of it's requirements. You may or may not need relayd if you only use the wireless for a single side (ie just the bridge back to the WD N900). I am of the opinion that when you connect the Pi to the WD900 that if you fundamentally set the connection to LAN (as opposed to WAN) and then bridge the LAN and WLAN this should work. Never tried .