Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, have you carefully checked that all the pins of the WU-1a Wireless Mobile Adapter are present and aligned correctly ? These pins have a peculiar arrangement as may be viewed HERE.
The above would be the only easiest user attempt as all others would be internal to the dongle.
Please click HERE. Key in "D7100" less the quotes in the space for 'Product name or number' and click on the X. Then click/select D7100. Displayed below would be a list of Nikon authorized Repair Stations with their contact information.