Digging around in the Arduino code, it seems that this MicroMod board is mapped in the [boards_remap.json file to the SFE_MMPB_STM32WB5MMG implementation. That can be found in [this directory. However, symbols like RX1 are not defined in the [relevant header file as they are in the equivalent [header file for the STM32F405 board.
So, I think someone needs to look at the schematic for the processor board and make a similar set of definitions to those for the STM32F405.](Arduino_Core_STM32/variants/STM32F4xx/F405RGT_F415RGT/variant_MICROMOD_F405.h at main · stm32duino/Arduino_Core_STM32 · GitHub)](Arduino_Core_STM32/variants/STM32WBxx/WB5MMGH/variant_SFE_MMPB_STM32WB5MMG.h at main · stm32duino/Arduino_Core_STM32 · GitHub)](Arduino_Core_STM32/variants/STM32WBxx/WB5MMGH at main · stm32duino/Arduino_Core_STM32 · GitHub)](Arduino_Core_STM32/tools/platformio/boards_remap.json at main · stm32duino/Arduino_Core_STM32 · GitHub)