Hi
Did the webuser of your installation has access to your share ?
I’ve a different settings (dedicated NAS for the storage and an Odroid for Nextcloud) and I first mount the NAS share in Odroid and then use the External Storage App to those local mounting point.
To give you a quick overview of the settings you can have a look at @riegerCLOUD really nice how-to .
This could give you some hints.