I will try to explain how ISPConfig works on dynamic IP address.
First of all you MUST have nameservers with static IP address. There is no way around it. And no, internal IP address is not enough for nameserver to work properly. You need static external IP address. If you can't get static external IP address from you internet provider, you have to set domain servers with some redirection service (such as no-ip.com) and use their nameservers for your domain.
What to do:
a) you have to set nameservers in your 123-reg account to nameservers given to you by no-ip.com (I have 123-reg account as well if you need help with that, give me a shout and I'll help you).
b) you have to set your dns entries in your account with no-ip.com and set their service to update your router IP address automaticaly (I do not have that account so I acn't helpyou with that but they will have surely some info on their website)
c) you should not set dns records in ISPConfig
d) when creating domain in ISPConfig, you have to set your IP address to 192.168.0.3, your nameservers on no-ip.com, ISPConfig on your server and your router will make sure that relevant website will be displayed
Best of all is to get static address from your internet provider if this is possible.
Only when unplugged.