A user equipment, UE, for handling radio access technology, RAT, capabilities, comprising: a storage device; and a processing circuit, coupled to the storage device, wherein the storage device stores, and the processing circuit is configured to execute, instructions of: transmitting a first request message via a cell of a first RAT to a first core network to register to the first core network; receiving a first reject message from the first core network in response to the first request message; disabling the first RAT capability and a second RAT capability in response to the first reject message; searching and selecting a cell of a third RAT of a second core network in response to the disabling, wherein the first core network is an evolved packet core, EPC, network and the second core network is a next generation core, NGC, network, or the first core network is a NGC network and the second core network is EPC network; transmitting a second request message via the cell of the third RAT to the second core network to register to the second core network, when selecting the cell of the third RAT; and receiving a second response message via the cell of the third RAT from the second core network in response to the second request message. a storage device; and a processing circuit, coupled to the storage device, wherein the storage device stores, and the processing circuit is configured to execute, instructions of: transmitting a first request message via a cell of a first RAT to a first core network to register to the first core network; receiving a first reject message from the first core network in response to the first request message; disabling the first RAT capability and a second RAT capability in response to the first reject message; searching and selecting a cell of a third RAT of a second core network in response to the disabling, wherein the first core network is an evolved packet core, EPC, network and the second core network is a next generation core, NGC, network, or the first core network is a NGC network and the second core network is EPC network; transmitting a second request message via the cell of the third RAT to the second core network to register to the second core network, when selecting the cell of the third RAT; and receiving a second response message via the cell of the third RAT from the second core network in response to the second request message. transmitting a first request message via a cell of a first RAT to a first core network to register to the first core network; receiving a first reject message from the first core network in response to the first request message; disabling the first RAT capability and a second RAT capability in response to the first reject message; searching and selecting a cell of a third RAT of a second core network in response to the disabling, wherein the first core network is an evolved packet core, EPC, network and the second core network is a next generation core, NGC, network, or the first core network is a NGC network and the second core network is EPC network; transmitting a second request message via the cell of the third RAT to the second core network to register to the second core network, when selecting the cell of the third RAT; and receiving a second response message via the cell of the third RAT from the second core network in response to the second request message.