Time Capsule as NAS

My main goal is to keep the TC as a router for my flat, and utilize part of the the 3TB Disk as a NAS without going through iCloud, so that i can access my working files and pictures from the remote via internet using my iPad (e.g. to sync my pdf from apps like GoodReader).

A few things to keep in mind:

1. Although the TC can be used as a file server, it is primarily designed for Time Machine backups of Macs.
2. The TC is NOT a true NAS device as it only can provide one NAS-like function and that is to perform as a file server.
3. You cannot partition the internal HDD in the TC for multiple purposes. The HDD would have to be removed (voiding the warranty), connected to a Mac using a drive caddy, and then, partitioned/formatted with Disk Utility. The only option would be to use Disk Utility to create the desired size disk image on the TC's HDD. Then mount this disk image on the macOS Desktop.
4. The TC supports Back to My Mac for remote access. In turn BtMM relies on your Apple ID & iCloud account. If you want to access the TC, without using iCloud, you will have to configure the TC to accept incoming requests via port mapping.
5. Your iPad doesn't need to access iCloud for GoodReader ... just an Internet connection to the Amazon Kindle servers ... which would be faster than accessing them from the TC's HDD from a remote location.

6. iPad has no ability to access files on the network drive without using an app. e.g. file browser. It used to be able to work over internet but Apple improved the security and it no longer has that ability. The issue is protocols. A Mac will handle BTMM access to the TC over the network using AFP. Apple do not have AFP available in the iOS.

The only way to get around this is as Tesserax pointed out using port mapping.. highly dangerous now with SMB and generally not workable with most ISP who block SMB ports. Or using a VPN router in front of the TC. But in that case you can buy a vpn router like Asus and plug a hard disk into it. Much better than using TC in the network.