It’s not that easy to get PPP over WLAN running in Ubuntu.
Network Manager lacks the option to choose a connection, but instead always tries to connect through ethernet. This works fine when you are actually plugged to a wall, but sometimes you connect to a LAN over WLAN and then need to establish a PPP connection to the internet.
There exists a tool called pppoeconf (sudo apt-get install pppoeconf) that sets PPPoW connections quite easily, but somehow always conflicts with NM and causes it to stop working properly. If this happended to you as well, here’s a way to fix NM:
For now, my method of establishing such a connection is using pppoeconf to create the the connection, then fixing NM and using gpppon (sudo apt-get install gpppon) to connect after I’m on WLAN. Maybe there is a better way to do this, because gpppon uses pon and poff scripts which in term look for connections in /etc/ppp/peers so it’s probably enough to add a conf file somewhere in there – without using pppoeconf.
Hopefully there will be a nicer way to do this sometime in the future…