If you're selling your Fire TV device or it won't boot properly no matter what you've tried, there's a remote shortcut that triggers a factory reset. Just press and hold the Back button and the Right ...
If your devices only support up to 4K at 60Hz, a "premium high-speed" HDMI cable (18Gbps, HDMI 2.0) is perfectly fine. Again, just avoid really old or thin cables. Early HDMI cables (HDMI 1.4 or older ...