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Could you please confirm which UART are using? the miniUart or the main PL011 uart (https://www.raspberrypi.org/documentation/configuration/uart.md) ?
If the miniUart it probably won't work.
Secondly, i'd suggest trying it with CTS and RTS connected, as this is how it is configured on Pi3, Pi3B+, Pi0W
Let us know if these changes make any difference.