A data transmission method performed by an access and mobility management function, AMF, network element, the method comprising the steps of: • obtaining (step 301) information about a first connection status of a terminal device in a first network, wherein the first connection status of the terminal device comprises a deregistered state, a data transmission idle state, or a state in which the terminal device is physically disconnected from an access network in the first network, wherein the terminal device includes either a customer premises equipment, CPE, or a residential gateway, RG, o wherein in a first alternative, the first network is a 3GPP network and a second network is a non-3GPP network, or, o in a second alternative instead of the first alternative, the first network is the non-3GPP network and the second network is the 3GPP network; wherein the obtaining the information about the first connection status of the terminal device in the first network comprises: ∘ obtaining the information about the first connection status of the terminal device in the first network according to a heartbeat mechanism performed between the AMF network element and the terminal device; • determining based on the obtained information about the first connection status of the terminal device in the first network, whether a duration of the first connection status of the terminal device in the first network exceeds a specified duration; • when determining that the duration of the first connection status of the terminal device in the first network exceeds the specified duration, restricting (step 302) a data transmission procedure of the terminal device in the second network. • obtaining (step 301) information about a first connection status of a terminal device in a first network, wherein the first connection status of the terminal device comprises a deregistered state, a data transmission idle state, or a state in which the terminal device is physically disconnected from an access network in the first network, wherein the terminal device includes either a customer premises equipment, CPE, or a residential gateway, RG, o wherein in a first alternative, the first network is a 3GPP network and a second network is a non-3GPP network, or, o in a second alternative instead of the first alternative, the first network is the non-3GPP network and the second network is the 3GPP network; wherein the obtaining the information about the first connection status of the terminal device in the first network comprises: ∘ obtaining the information about the first connection status of the terminal device in the first network according to a heartbeat mechanism performed between the AMF network element and the terminal device; o wherein in a first alternative, the first network is a 3GPP network and a second network is a non-3GPP network, or, o in a second alternative instead of the first alternative, the first network is the non-3GPP network and the second network is the 3GPP network; ∘ obtaining the information about the first connection status of the terminal device in the first network according to a heartbeat mechanism performed between the AMF network element and the terminal device; • determining based on the obtained information about the first connection status of the terminal device in the first network, whether a duration of the first connection status of the terminal device in the first network exceeds a specified duration; • when determining that the duration of the first connection status of the terminal device in the first network exceeds the specified duration, restricting (step 302) a data transmission procedure of the terminal device in the second network.