Moto G6 Xt19256 Firmware Link

fastboot flash partition gpt.bin
fastboot flash bootloader bootloader.img
fastboot flash logo logo.bin
fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot flash system system.img_sparsechunk.0
... (continue all sparsechunks)
fastboot erase userdata   # optional, wipes device
fastboot reboot

Inside the correct folder, find a file name containing XT1925-6 and the software channel (e.g., ALI_RETBR_9.0_PPSS29.55-37-6_subsidy-DEFAULT_regulatory-DEFAULT_CFC.xml.zip).

You flashed the wrong modem. Re-flash only the modem files from the correct XT19256 firmware:
fastboot flash modem NON-HLOS.bin

Senior members often upload mirrors. Search for threads titled "[STOCK ROM] Moto G6 XT1925-6". moto g6 xt19256 firmware link