How to connect a DSLR camera to an OpenWRT router?

You're not going to get this to work without the camera being connected directly to a computer via USB.

You need such as Capture One or Nikon's own Capture Control Pro 2 to set up the connection.

Low-end Nikons didn't have any wireless capability of their own until the D5300. The D5200 had a separate wireless transmitter you could buy. The D5100 has no wireless capability at all.

Even modern DSLRs are a long way behind 'smart' phones in connectivity terms.
They've only really since this year been generally useable as web-cams, after all the major manufacturers invested some time & effort in the pandemic to retro-equip any potentially connectible cameras with that functionality. Unfortunately, the D5100 is too old to even work under that structure - Nikon Webcam Utility (inc supported camera list)
One thing a DSLR is not; it is not an IP camera.
It doesn't have its own on-board networking. Some have HDMI out to transmit picture, but cannot be controlled back down that same connection.

Okay, so it's been two hard days, but I did it.

You need to install USB/IP to your router and computer (I guess win8.1 doesn't support it) and then connect camera to router.

Detailed description at: USB/IP.

Also this helped me