I have the same problem, and other people too. There is a thread about this issue.
People in this thread also say, that wiping the phone does not solve the issue.
I am in contact with the support since months, and they still try to find a bug fix. I guess it would be the best, if you contact them, too.
My workaround is: I bought a second SIM card with cheap traffic. So the FP3 is not consuming all mobile traffic on my main SIM, as it refuses to connect to wifi automatically.
This is annoying and costs money, but for me this is the only solution for now. The problem exists for a long time (since A13 update) and nobody knows when they will fix it (and I don’t have time at the moment to install Lineage and setup everything from scratch).