So if I understand correctly, you have the LT plugged into a laptop which has the internal 172.16.84.1 address and also into the LAN which received the 192.168.178.31/24 address correct ?
* From here, you mention that you plug it through a direct power source ?
What I get from here is that the 172.16.84.1/24 scheme is not accessible because the interface it is responsible for connecting to when plugged into a device through USB is not accessible as it's just a power source.
You should technically still have a LAN connection, however.
Also, if you're looking to access the turtle remotely you will want to look into setting up the Open VPN server either at your home on a Raspberry PI or on a router / networking device if it supports it. Hope I understood your hang ups here.