There are places where you should engage in cost savings, and those where you should buy the genuine article and not somebody else's knockoff copy of it.
A genuine Harris programming cable is really not that expensive.
Get it from New London Technologies and be done with it.
It'll work. If your radio is good, that is.
The 7100s are notorious for having issues with their programming/accessory connector. Look for a case crack at the speaker-mic locking lug. If there's a crack there, the connection between the main board and the programming connector is probably poor or not working at all. That crack indicates (1) a broken front cover, and (2) a loss of holding pressure on the zebra strip connector between the front cover internal contacts and the display PC board front contacts.