Get GPS coordinates of lost phone even if GPS was off when lost
I happened to have a Samsung phone, so you can use the FindMyMobile service, which can get you the coordinates of your phone even if your GPS was off when you lost it. However, you need to register in the web beforehand and authorize the Remote Controls in the menu Settings/Lock Screen and Security/Find My Mobile/[Log in with your Samsung details] and then activate Remote Controls. 

It would be good to know any alternative for those not having a Samsung though...