Managed to solve an issue where the Shares existing before OS3/OS5 upgrade could not be seen after upgrading to OS5 either in Web Access or via the My Cloud OS5 iOS app. Shares created after upgrade could be seen but not those created prior.
Solved by turning the Share Access/Mobile & Web Access slider button Off & On again for each pre-upgrade Share. That made them appear on Mobile App and Web Access on upgraded Cloud access platforms.
Hope this helps if you are having similar issues
MyCloudEX2Ultra 2 Bay NAS - Firmware 5.08.115
MAC Big Sur 11.1
Chrome Version 87.0.4280.88
My Cloud OS 5 4.12.1