WP7702 AT command interface over UART (header raspberry) on mangOH Red with external microcontroller

Hi, I try to make communicate mangOH Red with a external microcontroller.
I want the external microcontroller to power mangOH Red. Then I would like to make communicate my external microcontroller with LTE-M sierra chip over the TX and RX pins on raspberry header with AT commands.

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FYI : When I use COM port of mangOH Red to use AT commands, I get !MAPUART:17,16 for AT!MAPUART?

I cannot, so I wonder if it possible to make communicate an external microcontroller with LTE-M sierra chip with the AT commands over RX TX pins of raspberry header of mangOH Red ???

If anyone has an hint, I would appreciate