The "cidnum" is the "customer identity number" - where "customer" refers not to you but to the market, region or carrier the phone was originally made for. You can find it using a program called "fastboot", but since it is related to the region and carrier you can usually take a good guess if you know that information (unlocking or debranding doesn't change it, which is why I asked about the original carrier).
From what you said I think I've found one file which has a change of working. You can find the download link in
this XDA post, which will take you to
this page. There is one link there dated April 2016, which is the one you want: the RUU cannot downgrade the phone software, so the others which all come from 2015 will certainly not work on your phone (as I can see from an earlier post that you have 2016 firmware).
This XDA thread contains instruction for how to install the RUU zip file. This is in fact where I found the links above. There is another way of installing the zip, which involves using the "fastboot" command I mentioned above. If you'd prefer to do it that way (in involves using the PC command line) you can find the Windows version of the fastboot program at
this website, and I can find instructions for flashing in RUUmode using that.
Now a couple of caveats here: it's possible it won't work because the cidnum doesn't match, or because the software version is slightly older, though I think this file should be the one you need. In those cases no harm will be done. But if this does run it will completely replace the phone's firmware and erase all data from it in the process. This is why I suggested a few other things you could try using recovery mode first - they are quicker, simpler and safer (if anything goes wrong while reflashing the phone's software it could be left in a worse state - you should be aware of that before you start, and do not interrupt the process. Make sure it's charged before you start). And because we don't know what caused this problem we can't actually be sure that the RUU will fix it; if for example it's a hardware failure, e.g. part of the internal storage has failed, then reflashing the software isn't going to fix that. But if a factory reset doesn't fix it then reflashing the software is the only thing left that you can try yourself: otherwise the options are get a professional repair or replace the phone.
