Hi @Helenowen
You write that the father’s address in Münster is or was your intended place of residence. You also write that he has not responded to the Ausländerbehörde and is not cooperating with your plan.
Based on the information you provide, in my view you need to change your intended place of residence. The fact that you are still married does not make a difference in this regard. If you do not have someone else you know in Germany that you can at first stay with, then **** for temporary accommodation such as a hostel, hotel, short-term rental.
As part of the visa process, you are asked where in Germany you plan to live. However, there is not a requirement in your case to show that you have somewhere longer-term to stay. The requirement to have suitable accommodation is for other situations, such as a foreign person wishing to join their foreign spouse in Germany. As the parent of German children, this is not a requirement.
The authorities have some discretion on whether to issue the visa as you are moving to Germany with your children rather than joining them in Germany. You can be given a visa but if must be clearly shown that you will no longer have Nigeria as your usual place of residence. You should at least be able to show you have somewhere to stay at first. Otherwise they may have concerns about the children’s welfare, i.e. that you would arrive in Germany without anywhere for your children to stay, in which case the authority in charge of child protection (Jugendamt) would be contacted.
Best,
Éanna