Ok tried on an old phone with usb otg and it works for me:
- tried Seriaotg.aia available here; GitHub - rkl099/Appinventor-SerialOTG: Appinventor extension SerialOTG with BT/WIFI client/server support, examples of apps and Arduino code, ver 1.5 · GitHub ( 2023/01/05 )
- tried p10A_OTG_PRINT_LED.aia available here Serial OTG. Arduino CH340. FTDI. rkl099's Extension ( 2020/11/21 ), after initializing the port the first time I click “open port” appears a dialog asking if companion should connect to the usb device, then need to click again the button to open the port
In both cases with default settings dtr=rts=parity=0 ( no hardware handshake, no parity )
P.S.
sometimes, when phone goes in standby, need to power cycle the phone to make the app work again