Print unicode in ESCPOS_NET

I need to print a Unicode Vietnamese. and my Code:

string text = "Phiếu Tính Tiền";
            var encoding = System.Text.Encoding.GetEncoding("windows-1258");
            byte[] bytes = encoding.GetBytes(text);
            string isoString = Encoding.GetEncoding("windows-1258").GetString(bytes);

            var e = new EPSON();
            printer.Write(ByteSplicer.Combine(
                e.Initialize(),
                e.CodePage(CodePage.WPC1258_VIETNAMESE),
                e.PrintLine(isoString)
                
            ));

but the result prints: Phi? u Tính Ti? n why is that, can someone help me? thanks.