How can a father be convinced that his rejection of marriage proposals due to differences in nationality is unjustified?

The standard for compatibility in Islam is religious commitment and good character, not agreement or disagreement in nationality, as the Prophet, peace and blessings be upon him, said: "If someone whose religious commitment and character you approve of proposes to you, then marry him. If you do not do so, there will be tribulation and widespread corruption."

The guardian (wali) is a condition for the validity of the marriage, so try to convince your father and involve his relatives and people of religious standing to mediate with him. If he does not respond, and you believe he is unjustly preventing you from marrying, you may refer the matter to the Sharia courts, or to the Muslim community if there are no Sharia courts in your country.