Maybe it's the latest Fairphone OS Open update to 18.10 that made my phone only use Edge, but settings all look fine, even in dev-settings.
I just ordered a new SIM-card to see if this is the problem.
But at least K-9 Mail is working again, at least for the last five minutes. Let's see after 7 days, when the problems did occur.
One of the first Mastodon instances, there is no specific topic we're into, just enjoy your time!