If you know the exact path to the node then you can use
XmlNode.SelectSingleNode Method (System.Xml) | Microsoft Docs[
^]
Consider the following example
XmlDocument a = new XmlDocument();
a.LoadXml(@"<ns5:Envelope xmlns:ns5=""http://schemas.xmlsoap.org/soap/envelope/"" xmlns:ns2=""http://www.temenos.com/T24/ofs/Response"" xmlns:ns3=""https://soa.nbf.ae/Enquiry/"" xmlns:ns6=""http://www.w3.org/2001/XMLSchema"" xmlns:ns4=""http://www.temenos.com/T24/ofs/ResponseCommon"" xmlns:ns1=""http://www.temenos.com/T24/ofs/NBFDDSLOANLOOKUPType"" xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"">
<ns5:Body>
<ns3:T24EnquiryResponse>
<Status>SUCCESS</Status>
<EnquiryOutput>
<NBFDDSLOANLOOKUP xmlns:ns0=""http://www.temenos.com/T24/ofs/Response"">
<gNBFDDSLOANLOOKUPDetailType>
<mNBFDDSLOANLOOKUPDetailType>
<ID>MG1805700141</ID>
<CUSTOMERID>532324</CUSTOMERID>
<PRINLIQACCT>012001163238</PRINLIQACCT>
<DRAWDOWNACCOUNT>012001163238</DRAWDOWNACCOUNT>
<CURRENCY>AED</CURRENCY>
<AMOUNT>829500.00</AMOUNT>
<LOANSTATUS>LIVE</LOANSTATUS>
<OVERDUESTATUS>NAB</OVERDUESTATUS>
<DUEMONTH>202103</DUEMONTH>
<MONTHDUEAMT>7028.34</MONTHDUEAMT>
</mNBFDDSLOANLOOKUPDetailType>
<mNBFDDSLOANLOOKUPDetailType>
<ID/>
<CUSTOMERID/>
<PRINLIQACCT/>
<DRAWDOWNACCOUNT/>
<CURRENCY/>
<AMOUNT/>
<LOANSTATUS/>
<OVERDUESTATUS/>
<DUEMONTH>202102</DUEMONTH>
<MONTHDUEAMT>7135.61</MONTHDUEAMT>
</mNBFDDSLOANLOOKUPDetailType>
</gNBFDDSLOANLOOKUPDetailType>
</NBFDDSLOANLOOKUP>
</EnquiryOutput>
</ns3:T24EnquiryResponse>
</ns5:Body>
</ns5:Envelope>");
XmlNamespaceManager ns = new XmlNamespaceManager(a.NameTable);
ns.AddNamespace("ns1", "http://www.temenos.com/T24/ofs/NBFDDSLOANLOOKUPType");
ns.AddNamespace("ns2", "http://www.temenos.com/T24/ofs/Response");
ns.AddNamespace("ns3", "https://soa.nbf.ae/Enquiry/");
ns.AddNamespace("ns4", "http://www.temenos.com/T24/ofs/ResponseCommon");
ns.AddNamespace("ns5", "http://schemas.xmlsoap.org/soap/envelope/");
ns.AddNamespace("ns6", "http://www.w3.org/2001/XMLSchema");
XmlNode b = a.SelectSingleNode("ns5:Envelope/ns5:Body/ns3:T24EnquiryResponse/EnquiryOutput/NBFDDSLOANLOOKUP/gNBFDDSLOANLOOKUPDetailType/mNBFDDSLOANLOOKUPDetailType/DUEMONTH", ns);