@Lundeborg 

The wireless network name and passcode of the Deco E4 can be changed, you can access the Deco app to do that. 

For your second question, it relats to the device itself and the modem/Deco E4 both. First, with client connects to the Deco E4, cause it is a mesh network, it will recommend client to connect to the best point, while when the device connects to the sagem modem, the AP steering function will be decided by the modem/device itself. 

May it help.