Figuring out ISP Vlan ID, how?

[Phixsator]

So my ISP has just upgraded its network in the area where I live and done some chances that differs from their previous setup. They have a policy that you have to use their own router on the fiberbox and set that to bridge mode if you want to use your own router but I figured out that they were only running with a vlan tag of 13 on the WAN port so I configured my Asus router to meet this simple requirement and I could throw their trash router in the drawer and forget about it. Not anymore though. So how do I figure out what they have changed so I can adapt my Asus Router to it and once again be satisfied with my network?

Short description of running hardware
ISP Router is an Icotera i4850, they reset and haven't yet send me the new password they apparently felt was needed to set without telling me.
My own Router is an Asus RT-AX56U, most likely doesn't matter.
My PC is running linux but I'm willing to install Windows temporarily to get this issue solved.
The onboard NIC on my motherboard is a 2.5g Realtek, I have no idea if the vlan id is stripped from packets like it seems to be on windows(driver thing apparently)

 

[Phixsator]

Plug the WAN port on the ISP router into your PC, use Wireshark to capture all the traffic, see what VLAN tag is being used.

This won't work if the ISP router auto provisions itself using the untagged VLAN before switching to another one for client traffic, if that is the case you will need to put a managed switch between the ISP router and the ethernet port on the fibre handoff and sniff the packets that way.

Thanks for replying and sorry for the late response. I managed, after 5 attempts, to get a proper enthusiast on the support line who was perfectly fine slipping me the vlan ids for both the IPTV box I have and the internet. It turned out that the ISP provided router had semi bricked itself during their update so they didn't even know what account details to use to log into it.. that POGarbage has been returned and the fiber connection is zooming away yet again on my own router.