Use the standard Serial Wire Debug (SWD) Arm® CoreSight™ Debug Access Port (DAP) protocol to enter debug interface mode.
Before the external debugger can access the CPU, it must first request the device to power up and make sure that the appropriate power domains are powered up. This is handled using the built-in CxxxPWRUPREQ and CxxxPWRUPACK feature found in the DAP. As long as the debugger is requesting the debug domain or the complete system to power up, the device stays in debug interface mode.
Before connecting to the network core, check if it is in Force-OFF mode and not held in reset. To check if the network core is powered up, do an AHB read of AHB-AP 0 by targeting RESET.NETWORK.FORCEOFF (0x50005614) in the application core. If the readout is 0, the network core is not in Force-OFF mode. If the readout is 1, write 0 to it to exit Force-OFF mode.