How does your PC get internet now?
If it is in another room, presumably it is wired to the current router?
I already described that in the original post. It gets WiFi, but the signal is weakened by thick walls. It's fine for everyday use, but not for streaming HD video on the LAN.
Your entire logic is faulty if the PC is getting wifi now as its main supply. If the wireless signal to the PC is poor due to construction/walls etc, what makes you think that the reverse path will not suffer the same issues.....................
The logic is not faulty, because if the Mikrotik can be hardwired to the PC in bridge or whatever mode, then it (the Mikrotik) can also be taken outside the room in the hallway and mounted on a wall, on top of furniture etc to avoid interference. That way only 1 wall will stand in the way instead of 2 or 3. The signal will become much stronger.
Since you have no ethernet wiring, and no coax wiring to the rooms your are SOL.
Your best solution is simple. Get a laptop to use to stream to your set top box and put it in the same physical location.
Or continue to attempt to stuff a noodle up a straw.
How in the world is that a solution? I need a stable internet connection in every room. Placing all computers in the living room does not solve anything.