I've recently changed ISP and had to purchase a new router. We have HFC nbn here in Brisbane QLD.

The first 2 Netgear routers couldn't connect and a dlink ac1600 did, for about a week then failed. I then today repurchased  the ax50 and again, unable to get internet. Chatting with the ISP, we can get as far as showing the router having internet but we don't actually have it. 

We've been using an old optus provided router in between and that works fine although takes some fiddling to reconnect after trying another router. 

Any idea why the optus router works but not others? TIA