Austria

Liwest MAY be one of the last providers that allows a custom router (they are pretty customer friendly in general). It was allowed back when I lived in Linz, but that's many years ago. Better ask what the policy is now.

What I did was turn off all routing functionality on the Liwest router-modem (putting it in so-called bridge mode) and add your router in front of it, which works really well. That way the router-modem will passthrough all packets to your router, which in effect "replaces" Liwest's router with your own.

I'm sorry for the non-answser. It depends heavily on the ISP in question. In France, you are legally allowed to replace your router but the ISP doesn't have to give you configurations, so you can't really. Most ISPs nowadays authenticate their routers in some ways, and while for some of them you just spoof a Mac address, for some of them it's basically a non-starter. Also I suspect a router from Amazon doesn't have the capacity to spoof the needed things.

You should call them to enquire about their support for non-isp routers.

AFAIK they do not support custom routers. I still have my original router running in bridge mode and an edgerouter behind it.

You could ask them :)