However, text without embedded fonts is still not displayed correctly，
Correct, with out the Fonts embedded in the PDF, no PDF reader can guarantee that the PDF will look the same on every platform/device.
How to fix it.
Ideally the author of the PDF embeds the fonts.
Otherwise, you would have to embed the fonts, by processing the PDF on a machine that has all the required fonts. Please see this forum post on how to do that.
Also, if you update to newer versions of PDFNet that PDFNet.AddResourceSearchPath is probably an easier/better API to use over AddFontSubst functions.
https://www.pdftron.com/api/PDFTronSDK/dotnet/pdftron.PDFNet.html#pdftron_PDFNet_AddResourceSearchPath_System_String_