LineageOS is Android 7, Fairphone OS (both of them) are currently still Android 6.
When Fairphone OS (both of them) will be Android 7, at least Fairphone Open OS should have all those settings LineageOS has, too, and they will be officially supported. That was @Lidwien’s point (I think) …
… and the coming up Android 7 Fairphone Open OS is relevant for your original question because with officially supported OSes you could turn to Fairphone for solutions, and if they can’t find anything else, ultimately they would have to come up with something like swapping parts or your whole phone.
With LineageOS you don’t have that kind of support.
So what’s your priority?
That doesn’t quite match, does it? You are using a not officially supported OS supplying automatic nightly builds without a beta testing process before release.
That LineageOS works so well in general is a huge applaudable achievement of everybody actively involved … but there are no guarantees.
So, if somebody suggests using Fairphone Open OS instead, especially when it will catch up feature-wise to LineageOS hopefully reasonably soon, it’s only honouring your own priority and not sidetracking.
Back to the original question …
- How are your antenna connections? They could be loose.
- Are you on the latest LineageOS build already? (Settings - About phone)
- LineageOS bugs might get sorted out by entering them in the bugtracker.