When i read about Rhodesian and South Africa i get the impression that Rhodesia was significantly more restricted when it came to international relations than South Africa. Is that correct? In that case, why? They seem quite similar at that time.
I think there are two principle reasons:
Rhodesia was a British colony which unilaterally declared independence. Britain and its friends and allies did not recognize this declaration of independence, and therefore Rhodesia did not have international recognition as a state. There was really no bloc of nations that had any interest in supporting this, so they never gained any degree of international recognition.
The white population of South Africa was dramatically larger (maybe 20% of the population vs probably less than 5% in Rhodesia), of much longer standing (most Rhodesian whites were 2nd or 3rd generation immigrants, whereas there had been a significant white population in South Africa for centuries), and contained a native white ethnicity, the Afrikaaners.
Ultimately South Africa was viewed as a legitimate sovereign state (including generally by local opponents of white minority rule), whereas Rhodesia was not generally viewed as legitimate by anyone.