I have found the solution to this problem. I was running into the same issue and as it happens, windows keeps a default KMS server IP in its registry. So after a Windows key + R and regedit typed, go there:
If there is a registered KMS, which will have a serial number like '55c92734-d682-4d71-983e-d6ec3f16059f' for example, delete any child you find in it.
Then the /skms command will work as intended and register a valid KMS server.