I finally figured it out. To flash firmware on this board using ELRS configurator via uart is to connect the USB-TTL TX to the TX1, USB-TTL RX to RX1 to the board (this is because the silkscreen for TX1 and RX1 shows the FC side UART which is the opposite to the receiver side, since we want to talk to the receiver, we need to wire it accordingly.) Of course, we will need the USB-TTL ground to the FC ground and USB-TTL 5V to the FC 5v.
Hold down ONLY the receiver boot button and plug in the USB-TTL and you can select the correct port in the ELRS configurator and start the firmware flashing.
According to JHEMCU, the board with the BMI imu should support WiFi firmware flashing. The one I am working with is the MPU imu version so unfortunately it will not support that.