Welcome Hidehiko,
The RTL-SDR support in WD V2.x is very experimental and even I haven't used it in many months.
'wd -i' is used with baseband AUDIO input devices which is functional but to use it you will need to attach one or more USB audio input dongles.
In WD v3.x I am adding full support for RTL-SDRs and any other SDRs which have ;SoapyAPI' drivers with the primary goal of having WD support the VHF/UHF bands not supported by the Kiwi.
However I have foudn that the RTL-SDR drivers tune very inaccurately by WSPR requirements. When I program a GSPDO driven RTL-SDR to tune up 1 Hz, it may jump up 400 Hz or down 200 Hz.
I suspect that WSPR users of RTL-SDRs are able to fiddle with their tuning and get WSPR decoding to work, but I haven't been able to find a SW algorithm which can do that tuning adjustment.
So until I (or someone else) finds a way to work around that problem, I don't think RTL-SDRs are going to be useful with WD.
Rob