Make sure you have installed the latest My Cloud app for OS 5 (not the one for others), should be v 4.4.29 (2021-12-21) or newer. This could also be a bug in the Android app for My Cloud OS 5.
To check if your the file manager is working in Android, try installing File Manager Plus or Solid Explorer for Android, both have free versions and they should be able to read the LAN samba share from the My Cloud OS 5 and these file managers will ask you to open the file (in MS Office for Android) for example. If these file managers work and the MC OS 5 Android doesn’t, then it is probably a bug or deprecated features.
There are ways to use the two aforementioned Android managers from the internet to securely access the files in your local LAN for free, but you have to install the free Tailscale VPN on both the mobile and the NAS or have a Tailscale subnet router in the same subnet as the NAS.
If your NAS is more than a few years old and you are using it commercially in a business as you seem to be doing, you should really think about shopping for a new NAS or another brand of NAS that is more capable and reliable.
edited: looks like a deprecated feature as noted
