Hi R Paul, if I may, I think you need to connect serial console in master switch. In stack group, the IP address need to be configured on master switch ONLY, not on standby switch or member switch. So please configure management ip address on master not on standby since the control plane and data plane managed by master switch only. Hence  we cannot use standby switch to control or manage the switch.
I have 2 S4048-ON switch configured as stack. When login using ip address with connection type ssh the stack login as SWCluster(standby)# In the mode configure command is not supported. What command to use to go to SWCluster#  to use configure command. Thank you for your support. Paul      
S4048ON switch stack login as SWCluster(standby)# - configure