Okay this is going to sound extremely silly but it actually seems to have been the case with the capacitors in either the transformer or in the unit itself. I kept the thing unplugged for a good 24 hours and once I plugged it in the SIM responded as it should. (Thank Odin!) So the hardware seems to be okay although (perhaps?) a bit too persistent.And to think it took 3 weeks to figure this out. Embarrassing.

Thanks for your help Derrek.