Can't change permissions on a network share smb/CIFS

I'm sure this is simple for someone but I have tried for the last 2 days every piece of information from googling and still no joy! So I would really appreciate some help. I'm newish to the world of Linux but enjoying the change.

The Problem

I recently built a Ubuntu headless 16.04.2 LTS server to run Plex with the media living on my Qnap NAS. Following this guide I got things going pretty easily. So created the mounts like this:

mkdir /mnt/Video 
mkdir /mnt/Audio 
mkdir /mnt/Multimedia 
sudo apt-get install cifs-utils

Added the following lines to the /etc/fstab file

//192.168.1.15/Videos /mnt/Videos  cifs  credentials=/etc/samba/credentials,uid=1000,iocharset=utf8  0  0 
//192.168.1.15/Audio /mnt/Audio cifs  credentials=/etc/samba/credentials,uid=1000,iocharset=utf8  0  0 
//192.168.1.15/Multimedia /mnt/Multimedia cifs  credentials=/etc/samba/credentials,uid=1000,iocharset=utf8  0  0 

then ran the mount cmd

sudo mount -a

the shares mount fine, Plex could read the media and build a library.

Then I wanted to enable Camera Upload from my phone and that is when the fun began. Straight up it wouldn't work so I thought simple- change the permissions for the folder on the NAS to rw and bingo should work- Nope.
I can make and remove a folder from CLI but Plex still no rw access. Plex created a user called Plex so I tried to change the owner and permissions to plex using sudo chown -R plex /mnt/Multimedia but I get "changing ownership of '/mnt/Multimedia/files' : Permission denied. Tried sudo chmod 755 -R /mnt/Multimedia and I get the same response..

When I run a

ls -l /mnt/Multimedia

I get

I have since tried different mount options with no luck. Tried creating a group and making plex part of that group and apply the group and no luck.

Why is it so hard to change permissions on a network share?