been a while since I've been here as my setup has just worked, problem free, since I set it up

I'm back cos I need a bit of advice/help please.
I've just had FTTP installed today and (naively) thought I could just connect my existing router (wrt3200acm) to the ONT, and reconfigure my logins.
spent most of my afternoon trying to sort it - I'm currently online using the Zen supplied router (FRITZbox 7530ax) in a sort-of bridge mode to my gargoyle wrt3200acm.
I'd like to do away with the fritzbox (it's poor and something else to plug in
thing is, I need to be able to specify a VLAN ID to proceed but there's nowhere in the GUI to be able to do that - is there any way to do it with CLI?
here's some info I've got from my research and I'm working along the lines of. this is from an openwrt forum but gargoyle is forked from that, right?
"Okay. The installation was successful. I added the VLAN tag in the luCI GUI by using -> network -> interfaces -> devices and then adding a VLAN interface (802.1q) that was tagged to the WAN interface (eth1 in my case). Then I used -> network -> interfaces to change the WAN protocol to that of PPPoE and entered my credentials given by Zen ISP and under the device section changed the option from eth1 to eth1.911 (which is the WAN port with the valid VLAN ID). I had to edit the WAN6 interface (setup as a
DHCPv6 server) and choose the eth1.911 vlan tag again under the devices tab again. Hope this helps someone else in the future who is struggling to setup their device for the ISP.
so I'm just hoping to keep Gargoyle and my wrt3200acm - _any_ advice/pointers/guidance would be _very_ much appreciated.
Thank you
edit:
right, so, to sum up
for city fibre the customer's router will need to talk PPPoE on VLAN 911. can I do that somehow via cli? I'd really like to keep Gargoyle for the stability.
Thanks again