The REPEATER was not designed to be installed in series. 

A REPEATER repeat message from a sensor to the Sigfox network and so will interpret messages from another REPEATER as messages from the sensor. If you put in series 2 REPEATERS, the first one will repeat again the message from the second one, and the second one will repeat again the message, and then an infinite loop of messages will be formed between the two REPEATERs.