Hello !
Could you please help me with the reference on where you are seeing that for the ISP programming you need CTS and RTS ?
If you check the User Manual of the device you can find in the chapter 38.4 Physical Interface
The ISP protocol is implemented on USART 0, using DIO9 for RX and DIO8 for TX. The baud rate is 115200 with formatting of 8 bits per character, no parity bit and 1 stop bit.
Regards,
Estephania