Dtd validation with TextAsset xml gives exception

I am trying to load xml files and validate them on a DTD file that I've created. But an error is returned by the XmlDocument. I find it quite obvious why the error is thrown, but I would also like to know how to fix this.

XmlSchemaException: XmlSchema error: XmlSchema error: Specified external entity not found. Target URL is config.dtd . XML Line 2, Position 1.

And the code that performs the DTD validation:

 private void ReadConfig()
 {
     // load the file from resources
     TextAsset text = (TextAsset)Resources.Load(filePath);
     StringReader sr = new StringReader(text.text);
     sr.Read(); // skip BOM, Unity catch!

     // validate with a DTD file
     XmlReaderSettings settings = new XmlReaderSettings() { ValidationType = ValidationType.DTD, ProhibitDtd = false};
     XmlReader r = XmlReader.Create(sr, settings);
 
     XmlDocument doc = new XmlDocument();
     doc.Load(r);
 }

The xml file that is read contains the following first lines:

 <?xml version="1.0" encoding="utf-8"?>
 <!DOCTYPE Scene SYSTEM "config.dtd">

Why is the dtd file not found? That is because this config.xml is located in Resources\Configs\config.xml. The dtd-file is in the same folder, but cannot be referenced. Because the file was loaded through a TextAsset.

Question: How can I reference to the correct dtd file without changing the doctype in the xml-file?