I don’t recall using any channels where the replay button did anything but replay (in some fashion – some channels do replay differently than others.) However, if that button provides no function in any context (ie: won’t do anything at all in the roku channel, youtube etc.) then I’m guessing that button itself is broken/defective. If “new” means you bought it less than 30 days ago, I would swap it at the retailer.
So, we have had our streaming stick for over a year. The previous channel button worked up until recently when we had to reset the Roku stick (not sure why we had to do that but that's another story). So, when we reset the stick the volume and channel buttons worked but not the previous channel button. In order to reset the stick, we were pulling the Roku stick from the TV. Just out of curiosity we just unplugged the stick from the power source, and it worked. Can't understand why, but it does work now.
On my Hisense 50R6109 from 2019 it's the left arrows just above just above the NETFLIX button. I just bought a new Hinsense 50 UHD 4K and you are correct...that button no longer works on my remove. Now I took the new remote from the new TV and tried it on the older Hisense 50R and the button on, so, it's internal...they changed the function in the newer TVs.
I am assuming we are all speaking of the same function/button - the button with the arrow doing a U-turn on all my previous Roku remotes (I have 4) while viewing TV through (in my case) the Spectrum TV app returned you to the channel you were previously watching - my new 4K devices/remote does NOT do this - I was told it was to go back ten seconds in the program you were currently watching - but it does NOT do this when viewing the Spectrum TV app
So I searched up a manual for the “Hisense 50R6109” that @Southernborn mentioned and it does say of the instant replay button:
“Select models. Streaming programs that support this feature and Antenna TV channels: if Live TV Pause is enabled, jumps back a few seconds with each press and resumes playing.
Broadcast TV: If Live TV Pause is disabled, jumps to previous channel.
When using an on-screen keyboard: Backspaces in the text you are entering.
Smart Guide: Returns to the current day and time.”
That’s an interesting description starting with “select models” – select models of the Hisense 50R6109??? In any case, it would appear that they claim previous channel capability only in very specific cases: ie: broadcast TV and Live TV Pause is disabled, and maybe it also has to be a “select model”.
On the other hand, others mention the Spektrum TV app. It sounds like this channel also provides/provided its own previous channel functionality. Basically, once an app is running, I assume they can treat keys more or less as they choose.