Since i connected TV Tuner to my Monitor i faced this issue (when you have 1 set-top box and more than 1 TV) changing channels from other room is complicated
I searched lots of products they were expensive so again tried to find some cheap alternative so got DIY projects but that were made using aurdino, sensor, power adapter, mobile, programming.....
cost of project for me was Rs. 0
I made this 2 Years back and upgraded it 1 Year back (original was made without soldering and using double side tape)
This was proudly made by me without using anything other then remote and old wires.
Step 1: Old Remote
Take your old remote, my remote was broken but still working (and was lying idle as we were using new remote)
so i opened it inside out and removed its IR Led
and solder any thin wires you have
(wires used by me are so old that while removing its insulation they were breaking so i burnt its insulation to get inside wires)
Step 2: FIX IR Led
Now connect IR Led to wire and check your connection
keep in mind the sensor should be placed where it can send signal to set-top box, i initially affixed it on wall using double side tape. later after few months i found an old non working webcam so i opened it removed the camera and inserted the IR Led on to it
So now it looks like a surveillance camera to all and we know the truth
(use one remote to control TV placed with set-top box and this remote for 2nd TV when watching TV from any other room where other TV is placed)