Nikon D6 Reference Manual page 565

Problem
The connection is
unreliable.
[Connect to smart
device] and [Connect to
PC (built-in Wi-Fi)] are
grayed out and cannot
be selected.
If [Auto] is selected for [Channel] when you
are connecting to a smart device, choose
[Manual] and select the channel manually
( 0 931).
If the camera is connected to a computer in
infrastructure mode, check that the router is
set to a channel between 1 and 8 ( 0 933).
These options are not available when the
camera is connected via Ethernet or using a
WT-6. Select [Disable] for [Wired LAN/WT] >
[Wired LAN/WT functions] in the setup
menu ( 0 937).
Solution
Troubleshooting Built-in Wi-Fi
513