I have D6400 and previously had a repeater WN2000RPTv2 which was damaged by lightning. I also have an old N600 (DGND3700v2) which is currently unused.
The N600 supports an AP mode, but the only documentation that I can find regarding this suggests that the AP mode device needs to be connected to the primary device via an ethernet cable.
Is there any way to get the N600 to work as a wireless repeater without a cabled connection, similar to the WN2000RPTv2?