You can fiddle with settings on your router (different frequencies or channels to suss out interference, transmit power, etc.), but if that doesn't work, and given that you already replaced the router, you probably don't have many options. I haven't had good luck with repeaters either, but a second AP might do the job if your issue is simply distance.
- Run a really long ethernet cable across the house so the Roku wouldn't have to rely on wifi. This won't work in practical terms.
Some flavor of this is your only real solution. Wifi has come a long way, but it's still fundamentally garbage in the throughput/reliability department ("Everyone likes wifi until they have to use it."). If I had a Roku/similar, I would definitely have it on a hardwired network connection.
You can be creative with the wiring though. You can snake it through a vent, tuck it underneath something, or use that powerline adapter someone mentioned upthread. I once used an ethernet cable that went out a living room window and up a story to a bedroom window (I wouldn't recommend that long-term).