You are gonna have to setup logging, otherwise you are flying blind. Then grep pppd /var/log/system.log .
If you do not provide the information requested, then no-one can help you.
I did setup log,
Log output level: debug
Cron log level: debug
Log file location: /tmp/system.log
As I said
I run grep pppd /tmp/system.log
But nothing appears, what can I do?
Even setting up log as in the picture gives no result when running grep pppd /var/log/system.log
Share /etc/config/network to start with
Oh, I missed the fact that this is a DSL router. I have no experience with these, so I have no idea how to debug the DSL config.