I think there are more instances of routers. You can not for use ip-forwarding because your internal router as no world wide ip-address. Check your router intern and extern ip and look your world wide ip with e.g. https://ifconfig.me
If you really want use your pi from extern you need a system on the internet. then you can use e.g. remote port forwarding for access from intern to internet and in this tunnel back from internet to your intern network.