My subnet mask is 255.255.254.0
The gateway is x.x.128.1
The DHCP pool ranges from x.x.129.20 to x.x.x.255 (still in the same subnet)
My NAS had a static IP x.x.128.128 and the WiiM automatically got assigned an address in the x.x.129.x range.
I fixed the issue by bringing my NAS' static IP in the x.x.129.x range, but this shouldn't have been a problem! Seems like WiiM is not reading the subnet mask correctly. And why all of this even matters with a Plex Server boggles my mind. So far for ease of use