A User Equipment (UE) including a wireless transceiver and a controller is provided. The wireless transceiver performs wireless transmission and reception to and from a first service network utilizing a first Radio Access Technology (RAT) and a second service network utilizing a second RAT. The controller starts a Mobility Management (MM) back-off timer in response to a first MM procedure with the first service network being rejected, determines whether both the UE and the first service network support a dual registration mode in response to starting the MM back-off timer, and initiates a second MM procedure with the second service network via the wireless transceiver when the MM back-off timer is running, in response to determining that both the UE and the first service network support the dual registration mode.