Of course you can still use it as a Network Attached Storage device. It will work fine as a basic NAS. Because you have a first generation single by My Cloud running OS3 v4.x firmware, WD had terminated (in 2022) all WD supported remote access to the device. WD further recommends for security reasons to isolate the device from the internet.
My Cloud OS3 End of Service:
Customers with legacy, My Cloud OS 3-compatible devices
On April 15, 2022, support ended for prior generations of My Cloud OS, including My Cloud OS 3. There are critical security updates that are only available for My Cloud OS 5-compatible devices. If your device isn’t compatible with My Cloud OS 5, you can only access it locally. As of April 15, 2022, your device no longer receives remote access, security updates, or technical support. To help protect your content, we recommend that you immediately back up your device, disable remote access, disconnect it from the internet, and then protect it with a strong, unique password.
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Q: Will my device stop working if I can’t upgrade to My Cloud OS 5? Will I lose access to my data?
A: After the services are shut down, you can only access the device on a local network. Access through the My Cloud OS 3 mobile app or web app won’t work. Email notifications won’t be delivered. Devices that had auto-update enabled received a final firmware update that disabled remote access and outbound traffic to cloud services. Now, support has ended and you have to use your local network to access your device.
If you haven’t done so already, you may want to reset the My Cloud back to it’s default settings by performing a 40 second pin reset. A 4 second or 40 second pin reset is non data destructive.
Steps to Pin Reset and System Only Restore a Single-Bay My Cloud OS 3
Directions for how to map a My Cloud Share on a Mac:
How to Access and Map a My Cloud or My Cloud Home Network Drive on macOS
If the file transfer is slow that may be caused by various local network factors including what method of access (Ethernet or WiFi) one is using to connect their computer or mobile device to the local network. Also ensure the My Cloud is directly connected to the local network router/gateway and not through a Mesh Node or Ethernet / WiFi extender.
Also note that v4.x firmware is OS3 not OS5. There is a separate subforum for the discontinued/unsupported OS3 devices:
Legacy Products - My Cloud OS3