Creating code to identify which COM port various serial devices are connected to in MATLAB? And MATLAB App Designer?

Question

I am attempting to write a code which recognises which of my two serial devices are connected to which COM ports. This code will eventually be implemented in to a GUI on MATLAB's App Designer software. I have found a successful method of connecting to one of my serial devices at a time; this involves me listing all the previously taken COM ports and then asking my software to run through all the remaining COM ports until it finds one connected. I would then repeat this process for my second seral device however, as you can imagine this process involves the user staggering the USB connections made and in general it is more user input than I want.

Ideally, I would like the user to plug in both serial devices select one button on the GUI triggering a call-back which connects both devices. The code I have written below doesn't produce any errors and I have stepped through it line by line in MATLAB and it performs as expected, where I can write data to the serial devices. However, when I transfer it to the App Designer after the connection has been established I can't write/read any date to my device.

Below is the new code I developed. I have initially included the functions used and then titled the different call-backs from pushing buttons on the GUI:

function Startup(app):

%% Establishing which ports are already taken
% Produces a list of the COM port numbers that are unavailbale so that the
% app doesn't look for that one to connect to
taken_ports = serialportlist;
u = length(taken_ports);        
for l = 1:u
    taken_ports_2 = regexp(taken_ports(1,l),'\M','split');    
    taken_ports_3(l) = str2num(taken_ports_2(1,2));
end

function SerialPort(app)
            % Now plug in the USB and it will recognise the correct serial port
        for a=1:100
               if     a == app.takenports3 %if port is already taken it won't try to connect to it
                   app.LDS1000 = []; 
              elseif a~= app.takenports3
                   try
                      app.port_L=sprintf('COM%d',a);
                      app.LDS1000 = serialport(app.port_L,9600,'Parity','none','DataBits',8,'StopBits',1,'Timeout',5);
                      break
                   catch
                   end
               end
        end

function ND_SN(app)
            % Detrmining the connection to the ND280/LDS by asking the
            % machine what the SN Number is
            write(app.ND280,app.ESC,"char");        
            write(app.ND280,app.A0400,"char");
            write(app.ND280,app.CR,"char");
            
        end

        function Two_Connect(app)
           if app.ND280.NumBytesAvailable ~= 0 %This means that the serial number has been read and Port N is now connected to the ND280 
                  ND_port = regexp(app.port_N,'\M','split');
                  ND_Port_N = ND_port{1,2};
                  ND_Port_N2 = str2num(ND_Port_N);
                  app.takenports3(1,(app.u+1)) = ND_Port_N2; %Adding Port N to the taken_ports_list
                  % Configuring the terminator and the setting to interface mode
                  configureTerminator(app.ND280,'CR/LF'); %configure the terminator 
                  read(app.ND280,13,"uint8"); % Need to read it so that this data doesn't get in the way of redaing future data
             for ap=1:100
               if     ap == app.takenports3 %if port is already taken it won't try to connect to it
                   continue 
              elseif ap~= app.takenports3
                  try
                      app.port_L=sprintf('COM%d',ap);
                      app.LDS1000 = serialport(app.port_L,9600,'Parity','none','DataBits',8,'StopBits',1,'Timeout',5); %The LDS is now connected to Port L
                      break
                   catch
                  end  
               end
             end  
             configureTerminator(app.LDS1000,'CR/LF'); %configure the terminator
            elseif app.ND280.NumBytesAvailable == 0 %This means that the serial number wasn't read correctly and Port N is instead connected to the LDS.
                   app.ND280 = [];
                   app.LDS1000 = serialport(app.port_N,9600,'Parity','none','DataBits',8,'StopBits',1,'Timeout',5); %Connecting Port N to the LDS.
                   configureTerminator(app.LDS1000,'CR/LF'); %configure the terminator 
                   LD_port = regexp(app.port_N,'\M','split');
                   LD_Port_N = LD_port{1,2};
                   LD_Port_N2 = str2num(LD_Port_N);
                   app.takenports3(1,(app.u+1)) = LD_Port_N2; %Adding Port N to the taken_ports_list
            for ad=1:100
               if     ad == app.takenports3 %if port is already taken it won't try to connect to it
                   continue
              elseif ad~= app.takenports3
                  try
                      app.port_L=sprintf('COM%d',ad);
                      app.ND280 = serialport(app.port_L,9600,'Parity','Even','DataBits',7,'StopBits',2,'Timeout',1); %The ND280 is now connected to Port L
                      break
                   catch
                  end  
               end
            end
            configureTerminator(app.ND280,'CR/LF'); %configure the terminator 
           end
        end

Call-back for Connect button pushed:

ND280Conect(app)
ND_SN(app)
Two_Connect(app)