GreggB, you might want to take a look at this thread. Trying to get a brand new fire device to side-load may prove to be a challenge due to the newest operating system that Amazon had gone to on the newest units.
🔥 Shhhhh! These Fire TV Apps Haven't Been Blocked & Probably Won't Be
Also, if you do a search, here on the Insider, there are quite a few threads that can help to decide if you want to keep the Cube or if you want to move on to another device. Just go up to the magnifying glass in the upper right hand corner to open the search function. Best of luck!