The domain registries for .com and .eu require that every zone has two nameservers or at least ns1 and ns2 must have different IP addresses, even if they are located on the same server. So if you want to run a dns server for real domains, you need either two different IP addresses from your cable provider or you need a external server as secondary dns.
As a second request would be if there is any tutorial how can I share internet on the second network card that I will install so I can pass the internet to the rest of my house.
ISPCOnfig is nut made to run on a router. Then its better to leave ispconfig in the internal network if your cable modem is a full router and only forward required ports from the cable modem to the ispconfig server. For that scenario, you have to use xetrnal IP addresses in the DNS records then and internal IP addresses in the website settings (sites module). The translation between external IP and internal IP (NAT) is done by your cable modem then.
Running your own webserver is not a trivial task. So if you just start to host some websites, it might be easier for you if you use the dns server of the provider where you bought the domain names and just run the web and mailserver part on your local server. When you are more familar with dns and josting, then you can later switch your domains to your own dns server.