Finally figure out the gpio serial. I’m now connecting Ardupilot and pi through gpio. Old problem, bluetooth occupied it. There should be some setting in pi needed to disable the bluetooth but never mentioned. I thought APSync had setup for me but apparently it didn’t.
Pi-Connect Lite documentation tells you how to enable its serial in raspbian, but not in ubuntu.
To do this
Edit /boot/firmware/syscfg.txt then add
# Disable Bluetooth
enable_uart=1
dtoverlay=disable-bt
(For pi3, the dtoverlay should be something like pi3-disable-bt. But I didn’t test it)
Edit /boot/firmware/nobtcmd.txt then remove
console = /ttyAMA 115200
from the line, to disable serial shell.