In which file are the credentials / IP-Adress / Path stored?

[Nutul]

Has someone experience with using a device with fixed IP onto the device IN the DHCP-Range?

You may not want to do that.
Usually the routers offer you a range of addresses to be delivered by DHCP, and you chose static IPs outside such range...
On mine for instance I reserved the first 50 addresses - 10.131.120.1-50 - for DHCP, therefore from 51 to 254 can be used for static ones.
Setting a static IP in the DHCP range can result in two devices wanting the same IP, and you do not want that, right?