Mark-o

OK so it appears that the actual factory central locking is still working as it should. if you use drivers side locking part on the handle that should also operate all door lock.
The aftermarket wireless or RC unit doesn't need the factory procedure it should have its own, some have programming button on the unit which needs to be pressed/held for so long etc. This should be in the instructions included with those aftermarket units on how to pair set of fobs included. 
Remember how this Nissan central locking works:
Wireless/key less/remote interfaces with the drivers side door lock unit to operate locks on all doors including drivers. The factory unit includes also wiring to all door sensors to confirm that doors are closed.
Aftermarket units don't need that crap indeed it is hindrance especially in case of rear barn door!  Thats why aftermarket units only need six wires as we don't need to make checks if the doors are actually closed, that just Japs stupid overzealous checking in case you leave keys in the vehicle etc...
First question: is Mongoose unit working? (even with remote right next to it) if it is then try to obtain information how it is wired up or wiring arrangement for it. How many wires are connected?
 
Second question: have you paired Ebay unit fobs with the Ebay unit? Use intructions/info how to provided with the unit, don't use Nissan OEM instructions it will not work!
 
Third question: if you connected Jaycar unit as per instructions, did you pair the fobs as per included instructions?
This is the one I have and it works brilliantly ==> https://www.jaycar.com.au/remote-controlled-car-central-locking-system-with-2-keyfob/p/LR8839
The unit has learning button which needs to be pressed/held for 1 sec then pressing lock on each transmitter(fob) will pair them.
Did you do that?
Green and Blue wires are the only one needed....to lock unlock on this unit! Essentially if not concerned about indicators its just 4 wires.
Positive, Negative, Lock, Unlock,  and 2x Indicators
I removed glovebox on my car ST 2004, to check/confirm my wiring setup:
LR8839 side          Nissan side
Red                         Red/White     B+ 12V  (It means constant battery power)
Black                      Black              B- Ground/Earth
Blue                       Blue/Red        
Green                    Blue                
optional indicators:
Brown                  Green/Red
Brown                  Green/Yellow
According to my original post on other forum I might have swapped above Lock and Unlock wires so above list is working correctly.
I have cut those 6 wires on original Nissan plug off....
Then used 8 pin white plastic connector kit from Jaycar ==> https://www.jaycar.com.au/automotive-8-way-plug-socket-250-series/p/PP2069 

to make it nicer to connect to aftermarket unit existing harness ( I hate soldering wires) of which I had to cut a lot of length off! I only needed 0.5 m of those 6 wires. Rest I taped up away.
Cheers
 
LR8839_manualMain_76676.pdf

Good stuff, glad to be of assistance.
Regards

Mate, I don't think so as if it works via drivers door buttons.... You need to understand how the system works...
Did you read that thread I posted a link to?  It explains how the system works. Basically the remote unit just interfaces with/via drivers door control and remote relay. Did you actually pair the remotes as in supplied instructions?  If drivers door controls work then its only issue is the remote receiver.
As with any Remote control you're required to pair the fobs before they will work!

Just re-check that wiring is connected correctly... wiring diagram bellow. Bonus with aftermarket unit is that unlike factory unit there is no checking for doors properly closed which commonly an issue especially with rear door not closing properly with barn door sagging a little. Other then that it makes no sense why the Ebay unit doesn't work unless the signals are reversed....

I've replaced factory wireless receiver for keyless entry for exactly same reason with Jaycar kit, it worked right away.
I followed a thread on patrol4x4.com forum and added my 2 cents worth to the discussion ==> https://www.patrol4x4.com/posts/6963332/
B/R