Bugs can be reported here. Note that this is only for bugs for the snap, if you are using the debian package you should file bugs against the Ubuntu package.
If the bug can be reproduced, enabling the debug output before doing so is highly recommended.
When submitting the bug report, please attach system log coming from the journal:
* $ journalctl --no-pager > system-log
And the output of the following command:
$ sudo modem-manager.mmcli -m <N>
, being <N> the modem number as reported by
$ sudo modem-manager.mmcli -L