Zyxel drive is slow accessing the NAS326 from outside of home network

Hi, 

I bought a NAS326 product and i'm encountering one issue with it. While accesing it fron Zyxel Drive the speed is very slow and the contents of my shared folders sometimes do not appear at all. This issue does not manifest when i connect to wfi fi. I need to access my files remotely and this is very dissapointing. I'm using this as a router https://www.amazon.in/TP-Link-TL-WR841N-300Mbps-Wireless-N-Router/dp/B001FWYGJS . 

Do i need to do additional settings or could it be my router the culprit ? 

Thank you ! 

#NAS_Dec_2018