W10 upgrade impossible on R15 7520

Hi

I have bought a R15 trough Dell's shop 2/3 years ago. I've been very happy with it untill its RAM went down. It is time to refresh my laptop. So I've changed the DDR for a good one and I bought a SSD to replace the actual hard drive. As Windows gives windows 10 upgrade I went for that but it didn't work.
As I want a clean install I was not frustrated, I just should reinstall Win7 on the SSD, then upgrade to Win10. As I've lost my install disk I went for solutions gived by Microsoft.

Microsoft ask for the product key in order to allow Win7 download, but doesn't recognize mine as valid.
Plus I've looked in my Dell's account and I can't find my laptop in my product list.

It's like my computer doesn't exist for Dell or Microsot even if first thing I did when I received it was to register.


How can it be ?
What solution can be found ?

Thanks by advance.