Two things to check here:
First was the MT7697 built for serial transport. To check goto the GCC/feature.mk and check the SWI_SPI_ENABLE (should be = n) and the SWI_UART_ENABLE (should be = y).
Second insure AT port settings are correct by doing following:
- on the device run microcom -E /dev/ttyAT
- execute at!entercnd=“A710”
- execute at!mapuart=17,1