It has been removed from their app store due to complaints from copyright groups, so it was wiped when your TV did its latest update. I don't think there's an alternative you can use so you may have to look at using a Firestick or Android box with one of the IPTV apps available.
I have Smart STB on Samsung Smart when the tv wanted to do an update recently I just declined it. Having said that dont know if an update can remove an existing/downloaded app? unfortunately in TV settings you cannot permanently turn off updates (disable automatic updates - Google for instructions) you just decline when it comes onto screen. SO have a look if Smart STB is still on App Store its just same as a Mag.