when you use just the device IP, it normally does go to the built in device page (the internal web server)
Since Plex works, that confirms everything Plex is working
It does seem the WD management software isn’t working.
According to WD:
If your WD My Cloud OS5 web UI isn’t working, try a full power cycle (unplug and plug back in) and ensure you’re accessing it within the same subnet as the device. Also, check for VPNs or other network configurations that might be interfering, as some users have found that disabling firewalls and antivirus temporarily can help.
Is this OS3 or OS5 ?