I have two Samsung TVs (49MU6100 and 55Q80T) and this happen with both TVs:
The "Cast" festures only work when i use both TVs with wireless. If i use with LAN, a lot of Smart features doesn't work.
By my tests, seems like that something about the Cast/Mirror features are turned on when you turn the TVs wireless on.
So this happens:
1)Plug TV on energy,
2)Turn TV on with ethernet cable,
Smart apps work, but Smartthings and Cast/Mirror doesn't work.
3) Go to network settings,
4) Choose wireless,
5) Wait for the "wireless card turns on" and the list of availables wireless network apears,
6) Go back to network settings and choose cabled,
7) The TV will tell you that everything it's OK.
Now the Cast features works while using cabled internet by LAN (netflix or youtube on your phone shows the Cast symbol to manage the TVs app) and SmartThings too (the TV will show as "manageable" to change channel or volume on your phone).
When you use wireless internet on both TVs, the services works fine. When you use only cable, seems like the "Cast services" doesn't start until you do what i told above to "turn wireless on but not use it".
Both TVs are on last firmware available in Brazil (Q80T with v1462 and MU6100 with v1290)