I don't know the answer to your question but try looking around the modem-manager configs or see "man mm-cli", I think that's what handles the APN. You can also try to see if modem-manager-gui is available in your repos, it's a GTK+ app, you can set your APN details in there.
Disregard the above, @vkraven is right, I forgot Lomiri and Plasma use ofono and not modem-manager like phosh.
Disregard the above, @vkraven is right, I forgot Lomiri and Plasma use ofono and not modem-manager like phosh.