Thanks Eliot! Yeah noticed no ArduSub version wasn’t showing up after updates, good to confirm this is a signal for update failure! We have updated Pixhawk via direct USB connection to QGroundControl running on windows laptop. The update was successful, but the ArduSub version number didn’t show up on web interface.
So for our third experiment, we used Companion v0.0.28 + ArduSub v4.0.3 (or 4.0.2, the latest version). We flashed the SD card with latest Companion, and then inserted it to RaspPi. Then we disconnected ROV battery from board, connected Pixhawk to QGroundControl via USB, and updated ArduSub. After both are done, we rebooted the ROV, and only heard 3 beeps. We retried after setting up the Wifi, but sill only heard 3 beeps upon reboot.
So we are kind of scratching our heads about what else we could try.