Whatsapp Chat Migration

Last month I bought a new iPhone, but did't move the SIM in it until today, when I noticed that all the Whatsapp chats since the beginning of July until today were not transferred. I moved the SIM back in the old iPhone and made a back-up. When restoring the chat history from the back-up performed today, the messages of the last month are still not there. Now I cannot get those messages from anywhere and I desperately need them. What should I do? They should be captured in iCloud in the chat back-up of today, right? How can I retrieve them back? Thank you very much!