Assuming you have shell access to your device (barring that, that should work in KOReader's Terminal plugin), you might want to check if netcat can actually reach Calibre from your device.
e.g.,
nc 192.168.1.42 9090
If Calibre is actually setup properly and reachable from your device, it should answer fairly rapidly with something like
Code:
831[9, {"serverProtocolVersion": 1, "validExtensions": ["lrf", "rar", "zip", "rtf", "lit", "txt", "txtz", "text", "htm", "xhtm", "html", "htmlz", "xhtml", "pdf", "pdb", "updb", "pdr", "prc", "mobi", "azw", "doc", "epub", "fb2", "fbz", "djv", "djvu", "lrx", "cbr", "cbz", "cbc", "oebzip", "rb", "imp", "odt", "chm", "tpz", "azw1", "pml", "pmlz", "mbp", "tan", "snb", "xps", "oxps", "azw4", "book", "zbf", "pobi", "docx", "docm", "md", "textile", "markdown", "ibook", "ibooks", "iba", "azw3", "ps", "kepub", "kfx", "kpf"], "passwordChallenge": "", "currentLibraryName": "Calibre", "currentLibraryUUID": "6fc7c08a-4a8f-4833-8acd-f0a47d3aa201", "pubdateFormat": "MMM yyyy", "timestampFormat": "dd MMM yyyy", "lastModifiedFormat": "dd MMM yyyy", "calibre_version": [5, 9, 0], "canSupportUpdateBooks": true, "canSupportLpathChanges": true}]
The Calibre Wireless Device connection popup should be listing the actual IPs it's listening on, that should be pretty accurate on most systems.
The default port (9090) should also accurate, but might want to set it manually, just in case...
If nothing's answered (and/or the command actually fails and returns an error code), the problem lies outside of KOReader. And/or you have deeper network shenanigans at play.