As Chris @chrmhoffmann already stated above could you update to the current lineage and report if the problem persists? If yes, we would need a logcat with errors related to netmgrd.
Note: As far as I understand, there could be different behaviour if you install the latest official modem files provided from fairphone here after the lineageos install, but I think that this would be out of scope of lineageos and there surely will be other incompatibilities.