My Plex media server was working before adding a Mesh Deco network in Router mode. Once I did this my computer and smart tv could no longer access my NAS through the Plex web app or smart tv app. Plex would work but could not access my local NAS. 

The fix was very simple and I thought I would share. 

In your Sysnology Plex App do the following. 
1. Open the settings > top right wrench
2. Click on > Network in the left panel
3. Scroll down and find Preferred network interface > click drop down and select your IP for your nas. 

Initially my IP was set to ANY I had to change it to the DHCP address that was given by the TP LInk device. exsample: eth0 (10.0.0.5) Yours will be different. 

That fixed the issues I could then get my NAS content on my smart TV, web app etc. 

Hope this helps.