Something strange is definitely happening with pppoe clients.. this is not common but far from unheard of.
When you plug the modem direct to your computer are you running pppoe client to connect?
If that works then the pppoe client in the TC is failing with that particular modem.
Get a different modem and try. As rule of thumb match the chipset of the modem to the chipset of the dslam in the exchange you are linking to.. eg.. good brands are generally broadcom. cheap huawei are infineon.. a modem using the same chip can often work better than another brand.
Run the TC in the DMZ of the modem/router.. this doesn't quite alleviate the full effects of double NAT but helps a lot. Or half-bridge.. this is not so popular.. it has plenty of issues.
True 1:1 NAT is much like DMZ.. but fully forwards all ports.. what we term NAT is really NAPT.. one address to many.. a true NAT is a single public IP to single private IP.. all packets forwarded.
Bridge the TC and run the modem as router.
Unfortunately TC is very bare of options other routers have.. for example I have used pptp to pppoa relay on Thomson modems.. and it is brilliant .. stable and works great.