I have installed firmware-realtek package, then wlan0 is available.
My problem is that wifi link is not stable, I mean link is still up but I lose
network access. So I continue using 3.14.79+ kernel as there is
an older driver 8192cu.
thank you very much. I will try out
apt-get install firmware-realtek
Currently I have booted ubuntu with the kernel 3.16.60-28 and it is stable running the wlan.
For the old Raspberry Pi I have the same wlan adapter and it runs stable with kernel 4.14.78 and allkernels I used on the old Raspbeerry Pi.
I didn't even know that a special firmware-realtek exists. I thought that the kernel modules are enough to use the wlan adapter. I still don't know what this firmware does and if it's available e.g. for FreeBSD and freeBSD should run on the Raspberry Pis, too.
Anyway, firmware-realtek is exactly I was looking for and I will write my experience with it. Hopefully I don't need to downgrade the kernel.
For the kernel, has anyone tested the 4.14 or later kernel on Debian? How far is the progress in running the hardware with a 4x kernel on Odroid-C2? It's not important. I just ask for curiousity.