A 5G network having a multi-path transport protocol (MPTP) proxy external to the user plane function (UPF). The session management function (SMF) provides address information of the external MPTP proxy to user equipment (UE) and distributes access traffic steering, switching, and splitting (ATSSS)-related rules to the UE, the UPF, and the external MPTP proxy. The external MPTP proxy receives, from the UPF, (i) 3GPP uplink data transmitted by the UE via a 3GPP radio access network (RAN) and (ii) non-3GPP uplink data transmitted by the UE via a non-3GPP RAN, combines the 3GPP and non-3GPP uplink data to form network uplink data for a data network. The external MPTP proxy also divides received network downlink data into 3GPP downlink data and non-3GPP downlink data, and provides the 3GPP and non-3GPP downlink data to the UPF for transmission to the UE via the 3GPP RAN and the non-3GPP RAN, respectively.