I don't think IPv6 is turned on on the HG612 however so did wonder if that was the issue? Not sure how to test this?
The HG612 should only be used in bridge mode.
In this mode the only thing it handles is the connection between the modem and the cabinet.
It doesn't handle anything related to IPV4/V6 or anything routing related.
Make sure you have the HG612 set to "bridge" and not the routing setting.
Resetting it to factory defaults gives the config you want.
Basically no settings ever need changed on the HG612. Disabling QoS is the only thing that should ever be touched and even that isn't necessary.
In the right mode of operation the HG612 can't be the issue.