I’m pretty certain the last BT Hub my parents had (also plastic junk) was in modem mode and we’ve put it through a variety of routers. I was planning on upgrading them to Eero next time I’m at home so haven’t tested this properly yet (I did talk to their neighbour who said he has it working, but based on previous things he’s said I don’t know how much to believe!).
This is extremely disappointing if it’s not working for you, I don’t want to waste a weekend faffing around with things only to send it all back! Hopefully another UK based person can lend an eye/ear to your cause.