Device and method of handling an interworking procedure

A communication device for handling an interworking procedure, comprising:

• at least one storage device;
• and
• at least one processing circuit, coupled to the at least one storage device, wherein the at least one storage device stores, and the at least one processing circuit is configured to execute instructions of:
• registering to a fifth generation (5G) system (5GS) network and establishing Protocol Data Unit (PDU) session of a PDU session type to a Data Network (DN) identified by a first Data Network Name (DNN) and connected to the 5GS network;
• selecting a cell of an Evolved Packet System (EPS) network, after establishing the PDU session;
• determining whether the PDU session type is defined in a packet data network (PDN) type of the EPS network;
• determining whether the cell supports an attach without any PDN connection;
• and
• initiating an attach procedure on the cell, in response to determining that the PDU session type is not defined in the PDN type of the EPS network and the cell does not support the attach without any PDN connection;
• wherein the instruction of initiating the attach procedure comprises:
• transmitting an ATTACH REQUEST message comprising a PDN Connection Request message which comprises the PDN type of the EPS network;
• wherein the instruction of determining whether the PDU session type is defined in the PDN type of the EPS network comprises:
• checking whether the PDU session type of the PDU session is defined in the PDN type of the EPS network;
• and
• determining that the PDU session is not supported in the EPS network, in response to the PDU session type of the PDU session is not defined in the PDN type of the EPS network.