(I'm not sure if its appropriate to post it there. It's project sharing but I guess people in this category will benefit from this.)
Hi there. I'd like to share an open-source Serial Monitor there. It's called SerialTest.
It works like the Serial Monitor and Serial Plotter on the Arduino IDE, but it runs on Android, too.
Plus, you can communicate with your board via serial port, Bluetooth, BLE, TCP server, TCP client and UDP.
I hope you like it.