the 6400 is, IMO, quite a good low-price DIY system, but I don't consider it to be wifi. It is wireless.
The fault light indicates you should page through the log, note all messages down, and clear them. Can you do that? What messages are in the log?
unfortunately the control panel is highly regarded, and attracts good prices on ebay if you want a replacement.
Take the batteries out of all the devices in case one of them is doing something odd (each of them contains a transmitter). "Jamming" signals are very rare, but not impossible if you are close to a radio transmitter, an arc welder, a baby alarm or something. In this case the jamming will stop if you take the panel off the wall and carry it out of range, or plug it in at a friend's house. We have never had anybody on here with personal experience of a high-tech burglar deliberately trying to jam one of these alarms, though the hypothetical possibility is often raised.
It's possible there is an electronics fault.
You could try disconnecting the power supply, and turning off the backup battery (the switch if under a small white or grey rubber plug on the back which is removable) and see if it resets. If either the standby battery or the power supply are faulty, you can use replacements. I have no idea how to repair any other part. After disconnecting the power supply, the battery will keep it live for (IIRC) around 8 hours. It is possible to fit a replacement batttery; either a Yale original, or a Tamiya type which is cheaper. You will have to solder the connector if you use a non-original.
If not, you can do a factory reset (instructions are in the manual) but this will lose all its settings and you will have to learn-in all the accessories. if you do not have the manual I can look it up.