implementing buttons' functions on main instead on script from pyqt5

I created a GUI using Qt designer, and converted it into .py file. I used to write the functions directly in the GUI .py file, but I've been told to separate the code into two files: one with only the GUI commands as been converted from Qt designer, and Main - that would work all of the buttons and widgets of the GUI.

Now, I've managed to import the GUI from different file, and when I run the script the GUI does come up, but when I try to write functions I seem to have troubles.

This is my main:

from PyQt5 import QtWidgets
from mainwin import Ui_MainWindow
import sys


class ApplicationWindow(QtWidgets.QMainWindow):
    def __init__(self):
       super(ApplicationWindow, self).__init__()

       self.ui = Ui_MainWindow()
       self.ui.setupUi(self)   

       self.exitbtn.clicked.connect(self.exitclicked) ***this is the connection and function that I'm trying to make 'em work***
       def exitclicked(self,Dialog):
          sys.exit()

def main():
   import sys
   app = QtWidgets.QApplication(sys.argv)
   application = ApplicationWindow()
   application.show()
   sys.exit(app.exec_())

if __name__ == "__main__":
   main()

And I don't know if it's relevant, but this is my GUI converted from Qt designer.

from PyQt5 import QtCore, QtGui, QtWidgets

class Ui_MainWindow(object):
  def setupUi(self, MainWindow):
    MainWindow.setObjectName("MainWindow")
    MainWindow.resize(895, 422)
    self.centralwidget = QtWidgets.QWidget(MainWindow)
    self.centralwidget.setObjectName("centralwidget")
    self.txtoutput = QtWidgets.QTextBrowser(self.centralwidget)
    self.txtoutput.setGeometry(QtCore.QRect(150, 200, 471, 192))
    self.txtoutput.setObjectName("txtoutput")
    self.comboBox = QtWidgets.QComboBox(self.centralwidget)
    self.comboBox.setGeometry(QtCore.QRect(50, 90, 69, 22))
    self.comboBox.setObjectName("comboBox")
    self.lbl = QtWidgets.QLabel(self.centralwidget)
    self.lbl.setGeometry(QtCore.QRect(90, 30, 81, 16))
    self.lbl.setObjectName("lbl")
    self.lcdNumber = QtWidgets.QLCDNumber(self.centralwidget)
    self.lcdNumber.setGeometry(QtCore.QRect(340, 170, 151, 23))
    self.lcdNumber.setObjectName("lcdNumber")
    self.getserialbtn = QtWidgets.QPushButton(self.centralwidget)
    self.getserialbtn.setGeometry(QtCore.QRect(210, 50, 75, 23))
    self.getserialbtn.setObjectName("getserialbtn")
    self.chuckrealeasebtn = QtWidgets.QPushButton(self.centralwidget)
    self.chuckrealeasebtn.setGeometry(QtCore.QRect(50, 120, 75, 23))
    self.chuckrealeasebtn.setObjectName("chuckrealeasebtn")
    self.runbtn = QtWidgets.QPushButton(self.centralwidget)
    self.runbtn.setGeometry(QtCore.QRect(580, 70, 91, 61))
    font = QtGui.QFont()
    font.setPointSize(28)
    font.setBold(True)
    font.setWeight(75)
    self.runbtn.setFont(font)
    self.runbtn.setCheckable(False)
    self.runbtn.setObjectName("runbtn")
    self.textEdit = QtWidgets.QTextEdit(self.centralwidget)
    self.textEdit.setGeometry(QtCore.QRect(50, 50, 151, 31))
    self.textEdit.setObjectName("textEdit")
    self.stopbtn = QtWidgets.QPushButton(self.centralwidget)
    self.stopbtn.setGeometry(QtCore.QRect(680, 70, 91, 61))
    font = QtGui.QFont()
    font.setPointSize(28)
    font.setBold(True)
    font.setWeight(75)
    self.stopbtn.setFont(font)
    self.stopbtn.setObjectName("stopbtn")
    self.exitbtn = QtWidgets.QPushButton(self.centralwidget)
    self.exitbtn.setGeometry(QtCore.QRect(740, 370, 75, 23))
    self.exitbtn.setObjectName("exitbtn")
    MainWindow.setCentralWidget(self.centralwidget)
    self.statusbar = QtWidgets.QStatusBar(MainWindow)
    self.statusbar.setObjectName("statusbar")
    MainWindow.setStatusBar(self.statusbar)
    self.retranslateUi(MainWindow)
    QtCore.QMetaObject.connectSlotsByName(MainWindow)

 def retranslateUi(self, MainWindow):
    _translate = QtCore.QCoreApplication.translate
    MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
    self.lbl.setText(_translate("MainWindow", "Enter serial #"))
    self.getserialbtn.setText(_translate("MainWindow", "Enter"))
    self.chuckrealeasebtn.setText(_translate("MainWindow", "chuck"))
    self.runbtn.setText(_translate("MainWindow", "Run"))
    self.stopbtn.setText(_translate("MainWindow", "Stop"))
    self.exitbtn.setText(_translate("MainWindow", "Exit"))

if __name__ == "__main__":
  import sys
  app = QtWidgets.QApplication(sys.argv)
  MainWindow = QtWidgets.QMainWindow()
  ui = Ui_MainWindow()
  ui.setupUi(MainWindow)
  MainWindow.show()
  sys.exit(app.exec_())

The error that I get when I try to run it is:

AttributeError: 'ApplicationWindow' object has no attribute 'exitbtn'

Would love if someone can write the right syntax to be able to implement functions through main.

Thank you so much :D