Had a play with my HG612 3B today loaded with howling wolf’s (bcm96368MVWG_fs_kernel_HG612V100R001C01B030SP08_webgui) firmware for unlocking the BT HG612 modem and @Mrk26 is correct, I could not gain access to modem UI via Modem LAN Port 1, connected to RT-AC86U WAN Port – only via Modem LAN Port 2 with separate ethernet cable running from spare LAN port on ASUS router, 
I had forgotten there was 2 x LAN ports for this model, as and when you want to gain access to HG612 – I am sure @Mrk26 good instructions should work – Ignore my previous guidance 
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