GPRS config file

$ sudo cat /var/lib/ofono/*/gprs
[Settings]
Powered=1
RoamingAllowed=1
​
[context1]
Name=___ubuntu_custom_apn_internet
AccessPointName=pinternet.interkom.de
Username=fonic
Password=fonic
Type=internet
Protocol=ip

...
Take a backup, but if you are editing that file you might have to stop
ofono. Otherwise ofono will ignore the changes.
Maybe it's easier to execute “/usr/share/ofono/scripts/remove-contexts” and
reboot? You can use the terminal app on the phone.
We are currently working on the UI for this.