The message is:
A murder has just been committed at Salt Hill.
The first line of an early telegraph that helped catch a murderer. It was the first arrest ever made using a telegraph. The inventor of this cipher method is also the co-inventor of the telegraph.
The method of encryption is ...
... the Playfair cipher. The hint says "No J", which hints at an alphabet of 25 letters, where the regular English 26-letter alphabet has one letter omitted. Usually J is represented by I. These letters are arranged in a 5×5 grid. There are several ciphers that use such a grid, and the Playfair cipher is one of them.
The key is ...
... Charles Wheatstone. The cipher is named after Lord Playfair, who promoted its use, but it was invented by Wheatstone, a well-known scientist and inventor.
The key is used to transpose the alphabet: The square is filled with the unique letters of the key in order of appearance, followed by the unused letters. This gives the following square:
C H A R L
E S W T O
N B D F G
I K M P Q
U V X Y Z
The cipher ...
... encodes pairs of letters. If the message has an odd number of letters, the message is padded with an X. The cipher cannot encode pairs of the same letters, so these second letter is usually replaced with an X.
The decoded message reads:
AM UR DE RH AS GU ST BE EN CO MX IT TE DA TS AL TH IL LX
It turns out that the letter G is used as replacement for J.
The title is a clue because...
"No Cheating" is another way of saying "Play Fair."
This inventor created all sorts of cool things, including...
The English Concertina and many others.
...a timepiece especially useful at the North Pole
The Polar Clock
...and the earliest ancestor of the Oculus Rift.
He is most famous for co-inventing a means of communicating at long distances.