Setting up QNAP NAS NFS to access outside LAN

I currently have a QNAP NAS that I have configured so that I can access it outside my LAN via port forwarding, however this only directs me to the GUI of the NAS software.

I am trying to enable NFS so that I can be, for example, at university and coect the network drive via File Explorer -> Map Network Drive.

Currently on my NAS I have enabled NFS and port forwarded port 2049 on my router which shows as NFS. However if I try to coect to xx.xxx.xxx.xxx/Documents or xx.xxx.xxx.xxx:2049/Documents (which is a folder on the NAS which as * access to IPs) it says "The folder you entered does not appear to be valid. Please choose another." I set up a new folder and get the same message.

I am a total novice with networking and would be so grateful if anyone could help me get this working.

Cheers