You can go on for hours days and weeks to describe in short words what you can't believe.
If it nescsessary as a stress relief go on for some postings.
You can stay at assumings "this must work" and shaking your head. It is very unlikely that saying "it must work" or shaking your head will solve the problem.
Stop making assumings like "it can't be that"
Open your mind to check it all. Start with really analysing.
There is any kind of difference that makes the one thing work and the other not.
If you are interested in finding the difference a very careful and precise analyse will help.
Additionally if you want others to support you. You should provide a very detailed and precise description of what works and what works not
For this you should:
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describe each single little step from connecting your microcontroller to the USB-cable until sending the last command is finished
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post the complete sketch that works using the serial monitor
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posting character for character what you write into the send-field of the serial monitor and that you then send to the bluetooth-module
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posting what comes back as aknowledgement from the bluetooth-module
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post the complete sketch that does not work
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assigning each and every character to #defines "constants" or to arrays of char and use variables to send to BTSerial and to the serial monitor
If you think this is too much work for you. .... ----- .... ----- ..... ----- Well decide what ever you want.