There are a few things you can try, that I found in other community threads on the RMM3:
1) Make sure all RMM conections are made before AC is applied.
2) Flash the BMC code with the RMM installed.
3) Make sure there are no conflicting IP address on the network
4) Make sure BMC and RMM do not share the same subnet
5) RMM can be reset by restoring the BMC default configuration using SYSCFG tool and then reprogramming
The RMM3 also uses a lot of TCP/UDP ports. Have you tested with a client plugged in to the same switch as the S5520HC, so you're not going through a router first?