@Sebelution, it's worth reviewing the Nissan - e-NV200 sub-forum on this site to see if anyone else has found and fixed more or less your problem before. (I haven't. I am an e-NV200 "Combi" rather than panel van driver.)
If the keyless-entry button
is working on the other two doors, and reliability
doesn't vary depending on how far the key is from the door handle (where one of the pickups is), it's probably a fault in the driver's door button itself, or possibly the wiring to it. Might be worth changing the battery in the key, just to rule this out, since it's fairly cheap and they only last a year or two.
Next up in cost range is to get Leaf Spy (an Android, possibly iPhone, app) and a compatible ELM327 code reader dongle thing. Use this to read any fault codes (DTC, Diagnostic Trouble Code) from the vehicle and hope to spot one or two which might be relevant. There's an app called Car Scanner which can also read and clear DTCs, though Leaf Spy is more specialised for Nissan LEAF and works almost as well on e-NV200. Be warned that most ELM327 devices are poor fakes - the polite phrase might be "more or less compatible" - and don't work with Leaf Spy. If buying online, make it clear (i.e. a condition of sale) that you expect the gadget to be Leaf Spy compatible, and demand your money back if it isn't.
With a code reader, the amateur's approach is to look at the horrible long list of DTCs the reader reports, note that practically all of them are "archive" or "not active" status, meaning they could be months old and no longer relevant, think for a minute, then delete all codes. Any current faults will bounce back as newly-reported DTCs, and those are what you need to look up. Members of this forum can often help with diagnosis once you have current or recent codes to share.
DTCs show only things which the van's various computers have noticed being wrong. They don't find every fault!
Does the speed limiter work? Or does that have the same problem as the cruise control? They share most of their components, except that the cruise control relies on the brake pedal and stop lamp switches while the speed limiter doesn't use those but does use the accelerator pedal position.
As
@Rbrian notes, this might possibly be a 12V battery (the one in the motor compartment) problem. That's unlikely to affect just the driver's door keyless stuff, and very unlikely to affect the cruise control. I'd expect all kinds of problems starting the van, if this battery has failed. But if it's over 3 years old, probably worth the £60 to £100 for a replacement.
