I can no longer log into my WhatsApp. – Android Help

Hello everyone,

I hope someone can help me. Unfortunately, WhatsApp support couldn’t.
I logged in to my new phone with my Google account and then wanted to set up WhatsApp. To do this, I photographed the QR code with my old phone to establish the connection between the two and to transfer the contacts and chat histories. WhatsApp kept crashing on my old phone, which may have been because I wasn’t using WiFi with this phone, but was only online via a prepaid data SIM card. This meant that in the end, the contacts were visible, but no old chat histories. When I wanted to try again later, I could no longer log into my WhatsApp on the old phone, even when I uninstalled WhatsApp on the new phone (I hope that wasn’t a stupid idea…)
I thought it might be because I only had the prepaid SIM card in, but even when I reinserted the SIM card with the phone number I used to register with WhatsApp, the only thing that came up was the screen “Profile info: Please enter your name and optionally set a profile picture.” under which my old profile picture and my old name are shown and when I then click on “Next” it says “Connecting…”, then “Initializing.. Please wait” and nothing happens. Finally it says “Connection failed. Please try again later.” If I then click on “Cancel”, it still connects. Very strange..
How can I log back into WhatsApp on my old phone? Or get the chat history onto the new phone?
I have never manually made a backup in WhatsApp.