//Chrome
ChromeOptions options = new ChromeOptions();
options.AddArgument("--user-agent=Mozilla/5.0 (Linux; Android 11) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.77 Mobile Safari/537.36");
ChromeDriver driver = new ChromeDriver(options);

//Firefox
FirefoxProfile profile = new FirefoxProfile();
profile.SetPreference("general.useragent.override", "Mozilla/5.0 (Linux; Android 11) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.77 Mobile Safari/537.36");
FirefoxDriver driver = new FirefoxDriver(profile);