Here is datasheet for TL-PB20100: http://static.tp-link.com/TL-PB20100(UN)%201.0_datasheet.pdf But nor sure what's the pass-through charging?

Send email to fae@tp-link.com.cn for help. It should be a product in China, need to contact support in China for help. I remember they can reply you in English.

Find it at the back of the TL-WPA4220:p 8 numbers password

How about the lights on it? If the signal light is on, it means the extender has connected to the router successfully. The extender have same password with the router, that cannot be changed. Have...

At the back of the TL-WPA4220 you can find the default wifi password as well. It is Wireless Password or PIN which has 8 numbers:)

How about the lights on the extender? If you cannot see TP-LINK_Extender wifi, can reset it first(plug into the reset hole and press for 10 seconds until all the lights flash), then refer to this...