After talking with you it seems like you just need to get some wireless devices to re-enable communications between the computer and the sign.
You have the right idea to test the theory. Get a long Cat5 cable that you can run from the computer directly to the receiving card in the sign. 300m is the longest length of cable that will work. Power the sign and computer off and back on after connecting the cable. If you get a signal on the display you will know that the only issue is the communications devices between. Let me know what you find out. If you need further assistance, you have