Not sure if this would make a difference but the VLAN ID 101 is configured in the HG612 modem so should not be needed on the router, i.e. the Amplifi.
Is the HG612 running the unlocked firmware? The stats might give more information, but the modem is obviously working with a PPPoE connection from the laptop.
I thought Hardware NAT is a good thing to have as it speeds it up.
Is there any fault log available in the Amplifi which may give a clue as to what is wrong? It should work with a standard PPPoE set up.