Don't know of any way with L2TP. Don't think the OpenVPN Connect client does on-demand directly but you can install the Passerpartout OpenVPN client which does. Wireguard also has an on demand setting, As above, ZeroTier also does. For me the on-demand thing works great for browser and other apps but often doesn't pick up that the BI app needs Internet right away. You also can create a shortcut which will launch the VPN and then launch BI if using clients that support Shortcuts. But L2TP being built into IOS doesn't provide the hooks to do that.