Application modules (AMs) with multi-carrier subscriber identity modules (MSIMs) for diagnostic mode monitoring of signals within wireless distributed communications systems (WDCSs), including but not limited to distributed antenna systems (DASs). Related systems and methods are also disclosed. The MSIMs comprise circuitry configured to implement multiple SIM instances, each SIM instance containing carrier-specific data to enable the AM to register with a carrier to perform diagnostic mode monitoring of signals from the respective carrier. In one embodiment, AMs may be associated with components of a WDCS. By associating the AMs into components of a WDCS, live signals in the WDCS can be monitored and measured for monitoring the performance of components within the WDCS. The AMs may include one or more application level applications configured to receive and monitor signals in the WDCS, and to provide application-level information about such monitored signals to other components or systems, or technicians.