Mounting NFS share / how to access host root user

I would like to mount NFS share by modifying /etc/fstab as follow:

<local_ip>:/path/to/nas/dataset/folder /mnt/<folder> nfs ro,relatime,user,auto 0 0
However I don’t know how to do this, how do I access host system directly?
I tried using ‘Terminal and SSH’ add-on: running following fails with ‘permission denied’
mount -t nfs <ip>:<path> /mnt/
Directly changing /etc/fstab doesn’t carry over changes after reboot

Is there any other way to mount NFS in HASSIO?