Connect the SDA line from the RTC module to A4 from the Arduino. and connect the SCL line from the RTC module to A5 on the Arduino and then test whether it works. There have already been Arduino Uno CLONES where the connections in the PCB labeled SDA and SCL were not connected to the Atmega, controller, and therefore I2C does not work on these connections. However, the I2C interface is also available on the A4 / A5 connections (A4 = SDA / A5 = SCL).