Situation :

A device is configured to get its ip over a vlan that comes from the internet.
The modem has a port that makes this happen for the devices connected to it.

Problem :

Even with vlan tagging the device get its ip from the dhcp first thus not getting in the vlan and not working as intended.
There seems to be nothing wrong with the vlan setup except for the obtaining of the ip.

Question :

How do I make it possible that a device from wich I only know :
1. Mac adress
2. Port its connected to

to get it's ip from the unknown dhcp server that is hidden behind the incoming port?
Can I refrain the device from getting an ip from the lan's dhcp server?
Will it look further and get in the vlan as intended then?