Serial messages not received unless Arduino's serial monitor was first opened (Excel VBA to Arduino)

Projects Interfacing w/ Software on the Computer
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I tried different methods to try and circumvent the message reception problem on the Arduino side:

  1. Disabling programming mode by using a capacitor between the pins GND and RESET on the Arduino board, as suggested here and inspired by previous messages in the current thread : Preventing overwriting of sketch -- What is the best approach? - #11 by in0 and Programming Disable Jumper - #5 by Bskitter
    It could have freed up the com link for serial communication, but it does not work.

  2. Using the pins 0 and 1 as Tx and Rx to create a serial port that is independent on the built-in USB and its limitations. For some reason I can't get this serial port to work with my PC : the COM port is always busy, so I can't send anything on it from the PC.

For now I don't have more time for this project, so I implemented a functional but very unsatisfying solution: I use VBA to issue a Windows command to open the Arduino IDE, then keyboard shortcuts to open the serial monitor. Then I write the message using keyboard emulation in VBA, as a way to send it to the Arduino board directly from the serial monitor. Meaning that Arduino's serial monitor must always be open behind my VBA software.

Once again, thanks to all for your help!
And don't hesitate if you know what's up with the serial connection using pins 0 and 1 (Tx and Rx).