If you need to understand how to set up the Velop for wired backhaul, I found this (which you may already be aware of). It seems to want you to daisy chain Velop units together via the secondary wired Ethernet ports. I’m not sure if the two ports on the Velop constitute a switch in the Velop or if it’s a gateway setup, but either way we can just assume the non-modem port is the port we care about for a backhaul and use these instructions to come up with something that should work.
It sounds like one of your Velop ports is connected to the modem, and the other to a switch or device in a media cabinet. The wired backhaul instructions want you to plug the second Velop into that plug that is going to your cabinet.
If this is the case, you can:
1) If there is a switch in the media cabinet and it’s handy to use for the second Velop, just use that.
2) Buy a small switch, unplug the media cabinet from the Velop, and plug both the media cabinet and the Velop into this new switch.
In either of these modes you now have the Velop, media cabinet, and extra ports all on the same “layer 2” network. Now just plug the second Velop into a free port.
Don’t buy a router/gateway. You may be able to make it work but it’s an unnecessary complexity and will end up giving you multiple networks to manage.
Don’t buy a hub, I’m honestly not certain that real hubs even exist anymore. Even the cheapest ones I’ve seen will learn devices on ports and work like a switch.