Have you considered dismantling the phone again ~ well ~ and cleaning the contacts etc ~ again 
Though with so many sensors nor working it does tend to edge thinking towards a common problem, maybe a common point of the core module which no doubt has a chip for Inputs/Outputs.
But a reset would be useful, sadly, to see if it is just a software issue.
All the best.
I suppose that they could also update their phone software if it isn’t up to date. This may help as well…
Fairphone Support also suggested to check the build version. I checked and have the latest.
Meanwhile, they also answered my question regarding location of the sensors: “Unfortunately most of those sensors are located in the core module…”
I guess I know what I’ll be doing on the weekend… 
If you backup your phone successfully, I’d appreciate it if you told me the method you chose! I want to try something better than simply copying the data files to an external hard drive… .
Hi @Declan_Finney Welcome to the forum.
The original poster gave some detail of their problem, which has been responded to. They resolved their problem.
You don’t say how long you have had the phone nor how long after receipt this problem occurred.
It’s always a good step to inform Fairphone of any problems when they arise as warranties are not as valid if you either a) had the problem for a while and didn’t contact them or b) tried to fix it yourself, which may have compounded the issue.
So maybe you’d like to explain a bit more about your problem if you don’t find a solution soon?
What have you done to try and resolve the issue by the way?