S2LP register reading

I am debugging the routine(nanostack-border-router) of mbed OS. The hardware uses ST’s nucleo-f429zi and s2lp (steval-fki915v1).

When the borer-router is initialized, I call rf_read_register() to get the value of the register MC_STATE0 of S2LP. The value obtained each time is 0x00, but the default value of the register MC_STATE0 should be 0x07 according to the datasheet of S2LP.

Is this a bug in the rf_read_register() function?

How do I read the S2LP register correctly?