Slow Wifi read speeds

I have a MyCloud 4TB drive connected in my home network. I have tried to use this device as a backup area for my iPhone, iPad and MacBook Pro (Apple has seen me coming). I am struggling with the read and write speeds to the WD MyCloud. I have read a number of posts that state it is the network but that does not make sense to me.

Test 1
Wired connection from MacBook Pro to D-Link Dir 878 - ~70MB/s = 560Mb/s (Happy with this and it proves the WD My Cloud is using a 1GB LAN port.

Test 2
WiFi connect to router - ~4MB/s = 32Mb/s. Painfully slow

Test 3
Speed test Internet connection from MacBook Pro connected to the router via WiFi - ~80Mb/s = 10MB/s (This is close to what I pay in my package)

My question is, why are my speeds connected to the internet so much quicker than connecting to my WD MyCloud? I appreciate when doing backups I could cable the laptop to the network, but if that is the best solution, why don’t I just buy a Thunderbolt 3 hard drive and have speeds in the GB.