If you are absolutely sure that they are not visible in UIAutomatorviewer, you will have to tap on them "blindly". You can use percentage p.e.:
public static void tapPercentage(AppiumDriver driver, double x_percentage, double y_percentage) throws Exception {
Dimension size = driver.manage().window().getSize();
int xPoint = (int) (size.width * x_percentage);
int yPoint = (int) (size.height * y_percentage);
TouchAction touchAction = new TouchAction(driver);
touchAction.tap(xPoint, yPoint).perform();
}
Or you can use direct coordinates, but for that you should gather device density in order to calculate what are the proper coordinate to tap.