I address this question to the owners of the series J (with Tizen) and compatible with the "Second TV", feature that allows you to see on the screen of a smartphone, a tablet or a computer a different television broadcast from the one that you are viewing on the TV.
To be able to do so using a phone or a tablet you have install the "Smart View 2.0" app (the latest versions that do not have 2.0 in the name have not "TV to Mobile" feature) on a compatible device, such as the Galaxy S3 / S4 / S5 / S6 / Alpha / Note2 / Note3 / Note4 / Note Edge or tablet Note 8 "/ 10.1" / 10.1 "(2014 Edition) Tab2 7" / 10.1 "Tab3 8.
If you can not find the right app on Google Play or on Galaxy Apps store you can find it on apkmirror.com (version 1.0.25).
To see on the small screen a different channel than the one on tv, once enlarged to full screen video, you need to change the channel using the CAN keys (up or down) or the channel list and NOT instead the remote control function located at the top right, which needless to say it serves to change the TV channel.
The TV and the phone must be connected to the same wi-fi network.
With some phones it works also using WI-Fi Direct, but use a wireless router is better.