(This is assuming you are connected to your router by network cable as WiFi is not recommended for these TV's)
Power down the TV, pull the plug for 1 minute and power on again.
If this does not resolve, try restarting your router.
If still no joy, please go to the Smart IPTV app menu and try the following settings;
Stream Player Type - change from WEBOS to NC auto or vice versa
HLS Mode – If you have this option, trying turning it Off.
If still no joy, See here for our Router Reset Guide;
As a last resort, you can try reinstalling the app.