macOS: Preferences -> Network -> (+) Sign -> Interface: VPN / VPN Type: L2TP over IPSec / Service Name: {Desired Name} -> Create -> Server Address: {Server IP or URL} / Account Name: {ID} -> Authentication Settings: Password: {Password} / Shared Secret: {IPSec Secret} -> Advanced -> Send All Traffic over … -> …

ios: Settings -> VPN -> Add VPN … -> Type: L2TP -> Description: {Desired Name} -> Server: {Server IP or URL} -> Account: {ID} -> Password: {Password} -> Secret: {IPSec Secret} -> …

Win10: Start -> Settings -> Network & Internet -> VPN -> + Add a VPN … -> VPN Provider: Windows (built-in) / Connection name: {Desired Name} / Server name or address: {Server IP or URL} / VPN Type: L2TP/IPSec with pre-shared key / Pre-shared key: {IPSec Secret} / Type of sign-in info: username and password / User name: {ID} / Password: {Password} / Save -> …