NSA325 (v1) - Disk swap - NFS option gone - ZYPKG not retrieving from internet

I have just transferred data from an old 3TB disk to new 6TB disk and removed the old 3TB disk.  Now the NFS option is no longer available under Network menu, which I need. 

Thinking to reinstall it, I went to System Setting/Firmware/Package/Package tab but  there were no packages listed. So I clicked "Retrieve list from internet" but got the message "Currently, there are no packages. Please use "Retrieve List From Internet" to update package list." 

I thought to try manual install and visited ftp://ftp.zyxel.com/ (in Windows File Explorer) but it showed no folder for NSA325 (only NAS326, NAS 540, NAS542, NBG6604). 

I would be grateful for tips to get NFS back.  Thanks.