Local external storage only accessible in home network

Nextcloud version (eg, 18.0.2):
Operating system and version (eg, Ubuntu 20.04): FreeBSD/TrueNAS
Apache or nginx version (eg, Apache 2.4.25):
PHP version (eg, 7.1):

The issue you are facing:
I installed Nextcloud in a TrueNAS jail and attached a local external storage. I can see and use these data if I access Nextcloud via the internal server IP (192.168.x.x) but not when I connect to my nextcloud via my domain (cloud.xxxx.net) over the internet.
Sometimes I kinda see those files but I always get a small popup in the top right corner “This operation is not allowed” and I’m never able to access them. This might be a permission problem but I’m not really sure where to look.