Bought three of these. Put headers on two and tested. Both failed to initialize.
Tried with a Uno and a Teensy 3.1. Tried using Uno 5V and a separate regulated 5V. Tried 3.3V through 5V. No joy.

Contacted seller for guidance on how to troubleshoot and received a suggestion after a couple days to solder the VBAT pads on the back, making the part a 3.3.V (both power and I/O) only.

I assembled a level shift circuit and adjusted power to 3.3V and it worked.

OK, proves the regulator is foobar or at least something in the power regulation circuit. Can use as a straight 3.3V part but it's still a defect that the seller should address right?

Replied to the seller that there are 2 problems:
(1) this is listed as a 3.3-5V and should work as a 5V part but the units I received do NOT;
(2) the firmware is 1.26 not 1.28 as listed in the part description and the seller offered no guidance on how to update the firmware despite request.
OK, surely the seller will back the product right?

After waiting 6 days for a response (and sending a second request for response) to having seller refund or replace the defective parts, I get the reply that I should consider buying the V0 part and/or try using a Mega2560. Are you kidding me???!!
Buyer beware on this one.
I'll happily provide the entire communication thread on this if anyone is interested.

"So precious sophisticated module, we will casually ship? we are all tested before shipping. "

Then your testing is flawed. You yourself recommended the test to prove the faulty regulator circuit. Admit it...you shipped several units that are flawed. No biggie. It happens - you are not perfect. Just refund or replace the parts like any reputable seller would.

Thanks