Mobile Broadband dialog does not appear after "connect" or "mobile broadband settings"

[Peter Schüller]

After inserting a GSM modem into USB slot, it appears in the top right below the Wifi settings.

When clicking "connect" or "Mobile Broadband Settings", the settings dialog appears on the "Network" pane where there are settings only for VPN and Network Proxy.

$ lsb_release -rd
Description: Ubuntu 18.04.1 LTS
Release: 18.04

$ apt-cache policy modemmanager network-manager network-manager-pptp
modemmanager:
  Installed: 1.6.8-2ubuntu1
  Candidate: 1.6.8-2ubuntu1
  Version table:
 *** 1.6.8-2ubuntu1 500
        500 http://at.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
        100 /var/lib/dpkg/status
network-manager:
  Installed: 1.10.6-2ubuntu1.1
  Candidate: 1.10.6-2ubuntu1.1
  Version table:
 *** 1.10.6-2ubuntu1.1 500
        500 http://at.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     1.10.6-2ubuntu1 500
        500 http://at.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
network-manager-pptp:
  Installed: 1.2.6-1
  Candidate: 1.2.6-1
  Version table:
 *** 1.2.6-1 500
        500 http://at.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: network-manager-pptp 1.2.6-1
ProcVersionSignature: Ubuntu 4.15.0-42.45-generic 4.15.18
Uname: Linux 4.15.0-42-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Mon Dec 10 10:05:01 2018
EcryptfsInUse: Yes
SourcePackage: network-manager-pptp
UpgradeStatus: Upgraded to bionic on 2018-07-01 (161 days ago)