https://s.natalian.org/2017-08-31/l2tp.txt
I expected "Add Default Route" in the l2tp-client to do it, but it just stops my internet working until I removed the setting.
Why? Is it because there are two 0.0.0.0 default gateways here?

If it is because of the two conflicting routes, why doesn't the UI point that out? Why doesn't the UI offer at least to disable the 192.168.1.1 rule ?
Is there some firewall setting I am missing?