Using a custom Lock Screen does not unlock the SIM (PIN)
The system lock screen needs to be enabled in order for you to be prompted with the SIM pin. Unfortunately, 3rd party lock screens won't work with this feature unless the system lock screen is also enabled. One workaround would be to find (or write) an app that uses the 'disable your screen lock' permission constantly to ensure that the system lockscreen never appears but remains enabled in Settings, thus allowing you to use the SIM lock.