I use the Amplifi HD mesh wifi by Ubiquiti and it works perfectly fine. You can set it to 'bridge mode' and just have your current router manage the IP address allocation, routing, NAT, etc. You can also set it to 'router mode' should you prefer the Amplifi device to handle that stuff instead. It will do PPPoE, but it doesn't have a built-in modem.
I can't suggest a mesh system with a modem built-in, I've not heard of such a solution, sorry.