I have heard they'll let you use bridge mode, which sort of turns their Huawei box into a modem and lets you connect your own router up via PPPoE/ethernet cable.
There's also been mention of them offering a Unifi mesh option at install, if your house isn't getting great signal. So that's likely using a similar setup.
I now remember that they tentatively suggested I could plug my router into theirs' with the arrangement you describe above when I questioned them about the use of my own router.
They didn't offer any mesh system at the time of install, but my new higher speed, up from about 35Mbps with Plusnet, did highlight the shortcomings of the powerline set-up I was using around the house, so I went out and bought a BT Whole Home Wifi.
Unlike some "mesh" systems, your first node is not a replacement for your router, it merely connects by ethernet to the router and acts as the first link in the chain. You then add two more of the "dish-like" nodes around the house to complete the coverage and in fact at that point, you could turn off the WiFi in the router as the Whole Home WiFi performs all WiFi duties, including a guest network should you need this facility. So with this setup, I achieve around 700Mbps around the house, I think. I've never been quite sure of the best way to measure this.
There is a BT app which in theory helps with the set-up and placing of the nodes, but in practice, a quirk of it actually hindered things and I had to "trick" it into finishing the process. Apart from that, the app is useful and works pretty well for configuring some settings and also shows you what is connected to which node.