Ouch!
So coolant is leaking into the auto box heat exchanger and coming out of one of the transmission connectors?
That absolutely should not be happening.
You mentioned there were no blanking bungs. That struck me as a bit odd, if for no other reason than to prevent grime and debris out of the heat exchanger and contaminating the ATF. Assuming it was to be fitted to an auto. Are you certain the rad was brand new? Might it have been somebody else's faulty return?
I think the thread in the ports might be M14, but I won't swear to it.
You probably need a seal for each blanked port as well. Perhaps an oil pan drain plug and M14 Dowty washer might fit. The sump plugs might be short enough and will have a sealing land below the bolt hex. They should be easily available from any motor factor.
However I'd be more inclined to demand a replacement working rad, by courier, from carspares-direct. I just couldn't trust your radiator, even though you're not using the heat exchanger.