I use Times New Roman font for my report. But in the math expressions are not displayed Cyrillic characters.
Example:
\begin{equation}
S_{ном.} = S_{123}
\end{equation}
\documentclass[a4paper, 14pt]{extreport}
\usepackage[cm-default]{fontspec}
\setmainfont{Times New Roman}
\usepackage{titlesec}
\usepackage{indentfirst}
\usepackage{fncychap}
\usepackage{fancyhdr}
\usepackage[warn]{mathtext}
\usepackage[utf8]{inputenc}
\newfontfamily\cyrillicfont{Times New Roman}
\usepackage{polyglossia}
\setdefaultlanguage{russian}
\usepackage{amssymb,amsfonts,amsmath,mathtext,cite,enumerate,float, mathtext}