After 2-3 weeks of workarounds, trails, and tests; I discovered the fix. The issue was in the MTU value in the Router configuration.
My TV is LG Smart TV 43LJ550V with firmware updated; and it is connected to Internet via Wi-Fi (Modem/Router: Linksys X3500 and Range Extender: TP-Link RE200).
On my home Wi-Fi: LG Content Store is not opening (error is -324 network is unstable); NetFlix in not opening (error nw-2-5); while YouTube is working normally.
On my mobile Hotspot (only used for the sack of testing): All apps are opening and are working well.
I tried all what I found on the Internet as potential solutions but nothing.