This sounds a lot like Debian bug #922608 [1] which is fixed in Eoan.
Disco got modemmanager 1.10.0-1 on 2019-02-23, and Bionic got the backport on 2019-05-16.
Please try the version of modem-manager-gui from my PPA [2] and if this solves the problem we can prepare a Stable Release Update for Disco and Bionic.
This sounds a lot like Debian bug #922608 [1] which is fixed in Eoan.
Disco got modemmanager 1.10.0-1 on 2019-02-23, and Bionic got the backport on 2019-05-16.
Please try the version of modem-manager-gui from my PPA [2] and if this solves the problem we can prepare a Stable Release Update for Disco and Bionic.
[1] https:/ /bugs.debian. org/922608 /launchpad. net/~ginggs/ +archive/ ubuntu/ sru
[2] https:/