Look like you have'nt undertood what i wrote in my first reply.
Perhaps this explains better: Midi CC is sent from something and is recieved by something else. It only work when the midi CC is set to the same on both ends.
So if your controller is set to send a messages on "midi CH XX midi CC YY" then it will only work on reciveing units like in this case your software, if they have parameters or actions/ functions that are set to recieve on the same "midi CH XX midi CC YY".
This should tell you, that you broke this link between your controller and your software, when only changed the midi CC on you controller and got the result you got.
The point in using multiple midi channels is/ should be to seperate the same midi CC numbers, so they won't be the same messages anymore. So if you only change midi channel on your controller, then you also just cut the link between the sending and recieving end, like when you only changed the midi cc on your controller. So you still have to change the recieving end to recieve on the same "midi CH XX midi CC YY" that the sending end sends out, to get this to work.
In Reaper try to look at the Actions menu: Did a web search and found this:
https://www.youtube.com/watch?v=uaFnnaTKI_Q.
I think there are more videos about doing things like this out there. It has to do with Reaper.
And i still suggest you learn and get a understanding about midi CC.
Perhaps it will be easier, if you let the next and previous track functionality/ actions be controlled by your computer keyboard. But that's also a personal preference. It perhaps also depend on, if there are other possiblities in Reaper.