These error messages from the mmsdtng are caused by missing correct configuration entries in the file ~/.mms/modemmanager/mms. Mine is correct now for my provider FONIC and I do not see any error messages anymore from mmsdtng. Only success meessages like
purism@pureos:~$ sudo journalctl -b -0 | grep APN
May 06 09:47:30 pureos mmsdtng[787]: ../plugins/modemmanager.c:set_context() Current Context APN: pinternet.interkom.de, mmsd-tng settings MMS APN: pinternet.interkom.de
May 06 09:47:30 pureos mmsdtng[787]: ../plugins/modemmanager.c:set_context() You are connected to the correct APN! Enabling context...
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
May 06 09:47:30 pureos mmsdtng[787]: ../src/service.c:mms_service_set_apn() Service APN Set to pinternet.interkom.de
May 06 09:47:40 pureos mmsdtng[787]: ../plugins/modemmanager.c:set_context() Current Context APN: pinternet.interkom.de, mmsd-tng settings MMS APN: pinternet.interkom.de
May 06 09:47:40 pureos mmsdtng[787]: ../plugins/modemmanager.c:set_context() You are connected to the correct APN! Enabling context...
May 06 09:47:40 pureos mmsdtng[787]: ../src/service.c:mms_service_set_apn() Service APN Set to pinternet.interkom.de
purism@pureos:~$
Please read my post #14 in
APN settings values and storage