You should find a configuration file called "ppconfig" in /etc/ppp/peers/. It should contain the keyword "defaultroute", which causes pppd to create a default route with a metric of 0. You will need to remove or modify that route.
The ideal solution would be to have pppd create a default route with a higher metric. There was a proposed patch
to pppd which added a "defaultroute-metric" option, but I don't think it was ever accepted into the official pppd project.
You could change the "defaultroute" option to "nodefaultroute" and add an "ip route add" command to /etc/ppp/ip-up
, or you could leave "defaultroute" and use "ip route change" in /etc/ppp/ip-up
Of course, none of this will make any difference as long as you don't have an alternative default route through the wlan0 network.