I mounted a Box directory on the router via rclone. Its contents can be listed when I SSH in.
On the router, I have a samba share configured for that directory with guest access = OK.
If I try to access the samba shared directory from any app on my Android phone, I can see the directory ... but the single file in the folder can't be opened. I get a read error.
First confirm that the share works correct on Windows.
Android has no native samba client so every app that can access a samba share uses its own "logic". I usually test with "ASTRO File Manager" and "X-plore File Manager" on Android, which should work fine for a openwrt based samba share.