Morning
First you will need the location of all the units connected to the bus www.realoem.com will help. I may be teaching Granny a bit but a Fibre Optic bus is in simple terms a ring main the signal travels in one direction around it so if a unit is down/incompatible the signal stops there, The signal generally will start at a Bus Gateway and travel through each unit and on to the next until it is received back at the Gateway. When you unplug the Fibre optic connector you should see a flashing light indicating the signal is there, some units have a test lamp which flashes when the fibre optic is plugged in some don't so unplugging the next unit in line will reveal either a flashing light. If no light suspect the previous unit. There are bypass/test plugs available so if you find a dead unit plug in the bypass and check the next inline. Laborious but basically simple to do.
Several manufacturers use optical systems these days so there are bound to be "How Too's" on You Tube or similar, Google is your friend
The key fob could be as simple as the keys need pairing to the car or the antenna is faulty.
Dave