You have to handle the namespace in your code, e.g.
XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
nsmgr.AddNamespace("x", "urn:schemas-upnp-org:device-1-0");
string deviceType = doc.DocumentElement.SelectSingleNode("./x:device/x:deviceType", nsmgr).InnerText;
Please note the
XML
document you posted in invalid: there is a spurious deviceType closing tag near the end:
</serviceList>
</deviceType> <==== ERROR HERE
</device>
</root>