Вызвать функцию при выборе конкретного значения в comboBox

НИКОГДА НЕ ИЗМЕНЯЙТЕ код, сгенерированный Qt Designer, НИКОГДА.
Создайте другой класс, который наследуется от соответствующего виджета, и используйте созданный класс для его заполнения.

# ??? import serial
# ??? import time
from PyQt5 import QtCore, QtGui, QtWidgets, QtSerialPort

    
class Ui_MainWindow(object): #само окно
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("Датчики")
        MainWindow.resize(654, 404)
        MainWindow.setFixedSize(654, 404)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        
        self.graphicsView = QtWidgets.QGraphicsView(self.centralwidget)
        self.graphicsView.setGeometry(QtCore.QRect(20, 220, 181, 131))
        self.graphicsView.setObjectName("graphicsView")
        self.graphicsView_2 = QtWidgets.QGraphicsView(self.centralwidget)
        self.graphicsView_2.setGeometry(QtCore.QRect(240, 220, 181, 131))
        self.graphicsView_2.setObjectName("graphicsView_2")
        self.graphicsView_3 = QtWidgets.QGraphicsView(self.centralwidget)
        self.graphicsView_3.setGeometry(QtCore.QRect(460, 220, 181, 131))
        self.graphicsView_3.setObjectName("graphicsView_3")
        
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(460, 60, 141, 51))
        self.pushButton.setObjectName("pushButton")
        self.pushButton_2 = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton_2.setGeometry(QtCore.QRect(260, 60, 141, 51))
        self.pushButton_2.setObjectName("pushButton_2")
        
        self.comboBox = QtWidgets.QComboBox(self.centralwidget)
        self.comboBox.setGeometry(QtCore.QRect(50, 60, 141, 51))
        self.comboBox.setObjectName("QtWidgets.QComboBox")
        
        self.label = QtWidgets.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(80, 160, 71, 31))
        self.label.setObjectName("label")
        self.label_2 = QtWidgets.QLabel(self.centralwidget)
        self.label_2.setGeometry(QtCore.QRect(300, 160, 71, 31))
        self.label_2.setObjectName("label_2")
        self.label_3 = QtWidgets.QLabel(self.centralwidget)
        self.label_3.setGeometry(QtCore.QRect(520, 160, 71, 31))
        self.label_3.setObjectName("label_3")
        
        self.lcdNumber = QtWidgets.QLCDNumber(self.centralwidget)
        self.lcdNumber.setGeometry(QtCore.QRect(80, 180, 64, 23))
        self.lcdNumber.setObjectName("lcdNumber")
        self.lcdNumber_2 = QtWidgets.QLCDNumber(self.centralwidget)
        self.lcdNumber_2.setGeometry(QtCore.QRect(290, 180, 64, 23))
        self.lcdNumber_2.setObjectName("lcdNumber_2")
        self.lcdNumber_3 = QtWidgets.QLCDNumber(self.centralwidget)
        self.lcdNumber_3.setGeometry(QtCore.QRect(510, 180, 64, 23))
        self.lcdNumber_3.setObjectName("lcdNumber_3")
        
        MainWindow.setCentralWidget(self.centralwidget)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)
    
        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)
    
#        self.add_functions()
    
    def retranslateUi(self, MainWindow): #названия
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton.setText(_translate("MainWindow", "Закрыть"))
        self.pushButton_2.setText(_translate("MainWindow", "Открыть"))
        self.label.setText(_translate("MainWindow", "Temperature"))
        self.label_2.setText(_translate("MainWindow", "pH Value"))
        self.label_3.setText(_translate("MainWindow", "EC Value"))
    

class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
        
        self.comboBox.addItems(['Выберите  COM-порт', 'com1', 'com2', 'com3',])
        self.comboBox.currentTextChanged.connect(self.combobox_current_text)
        
# ???    def add_functions(self):                          
        self.pushButton.clicked.connect(self.closing)
        self.pushButton_2.clicked.connect(self.open_what)
        
#    def combobox(self, number):
    def combobox_current_text(self, text):
#        if ui.combobox.currentText()=="com1":
        if text == "com1":
            print(f'Ok, вы выбрали {text}. Делайте что-нибудь.')
    
    def closing(self, number):
#        sys.exit(app.exec_())
        self.close()

    def open_what(self):
        print(f'Откройте что-нибудь.')
  
  
if __name__ == "__main__":    
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())