== 3.3V vs 1.8V UARTs ==
Most phones expose their UART pins on 1.8V voltage, and most USB-UART converters expect UART side to have 3.3V. This works well for receiving characters from device; this is enough to read bootloader logs or kernel boot logs (dmesg). However sending 3.3V to device that expects 1.8V is dangerous; so you can't use shell over UART without an additional effort.
You'll need a voltage divider at least on converter->device TX line: use this answer https://electronics.stackexchange.com/a/333230
== See also ==