I had to review the posts above to figure out what problem we are addressing. The solution I proposed in the post above yours was not to address your aspect ratio issue, but rather my 4K TV's lack of HDMI inputs. So... The adapter you bought (AV to HDMI) is going the wrong direction.I just bought something like it, a RCA AV to HDMI HD Converter Composite Video Adapter with a HDMI Output and it didn't work is there any point in buying a similar one with a HDMI Input instead because they do make those?
You want an HDMI (or component) to AV composite converter, assuming it puts out anamorphic widescreen. Others have stated above that current Dish receivers no longer put out anamorphic widescreen on their AV outputs, so those outputs are a non-starter.