I think the built in component is text oriented so it can̈́t transfer any byte value 0-255. It is also limited in the type of usb-serial adapters it can use. Ref Using serial with FTDI.
If you are using my extension GitHub - rkl099/Appinventor-SerialOTG: Appinventor extension SerialOTG with BT/WIFI client/server support, examples of apps and Arduino code, ver 1.5, you have two ways of detecting incoming data, read a byte and test for an error or use the method Available(). There is no event handler in the extension, so you have to use the timer to poll the data. (If you use a loop the UI will be blocked)