issue some how solved after giving some delay in the uart receiver callback of stm32.
Does debugger provides some delay to the programming??
Timing is critical with embedded systems. A debugger session usually adds a breakpoint at the beginning of your main. Then it waits until you click run or hit F8. And your board is already powered on when flashing firmware at the start of your debug session, so there is additional time before running compared to power cycle without debugger. In addition you may have some variables in your live viewer which might force the debugger to periodically pause the MCU to read some variables. All these things affect timing.
Have you attached a logic analyser? I would look at the power, TX, RX and reset pins of your Modem and then compare the working scenario to the non-working scenario. It usually needs some time after reset before it can respond (read the datasheet of your modem for proper power-up, initialization and power down routine). Also if you are sending data before it is powered on you are phantom feeding the modem through its RX pin. If that is happening you can solve this by initializing your MCU TX pin after the Modem powered on, or add a series resistor between your MCU TX and Modem RX. Or use an open drain configuration (pullup to the switched power and open drain output setting for MCU TX).
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