Problem
Ethernet RTL8111/8168/8411 is not recognized by kernel 4.4.39-rt50.
Running ifconfig shows only the other network interface and lo.
Solution
It turns out the realtime kernel does not have module r8168. Instead it contains r8169.
However, by installing r8168-dkms the r8169 module is automatically blacklisted. This can
be fixed by
sudo apt remove r8168-dkms
sudo rm /etc/modprobe.d/r8168-dkms.conf