I have the same equipment and I am also having a similar problem.
The DSL link is very stable but if my ISP disconnects the PPP link for whatever reason , then the router does not reconnect the PPP link automatically. I have to log in to the router and click Disconnect and then Connect and then everything works OK.
This is a problem because I can't expect non-technical people to sort this out if I am not present. I may have to go back to my old Netgear router which never suffered from this problem but was replaced because it did not have wireless N.