How can I unlock my iPhone? It is locked to another network but it's also on activation lock?
It was my grandads but sadly he died?
Take the iPhone to an Apple Store with a proof, that you inherited the iPhone.
Turn off Find My iPhone Activation Lock - Apple Support
The activation lock is a security measure to prevent stolen iPhones from being activated and it cannot be disabled without the help of the previous owner or Apple, if you do not know the password.