If you want to upload more than 1GB to your WD EX2, the best way is to plug it straight into your computer (ie not into the router, so you won’t rely on WiFi but rather a direct connection).
Then plug the hard drive you want to transfer the data from into your computer as well (instead of into the back of the EX2).
Open the WD My Cloud app.
Now drag and drop.
It’ll still take a while and speeds can seem less than ideal but this should work (has for me). When I tried vie WiFi, at some point my computer would freeze and it would lose the WiFi signal.
The method described above has worked consistently, even though its not ideal.