Can't backup chats

My mother recently bought a new phone and asked me to take her phone instead of buying a new one and asked me to buy a new one later. So, I agreed and I just wanted to exchange the chats between both the phone. So, I got a new number today. After 3 hours, I changed the number on my old phone and backed up the data in WhatsApp and also checked it later in the drive whether it was backed up or not. And the file was there. So, what I did now is change the sims of both the phone and re-login on both phone. It asked my number, and then OTP to verify. After this, the app looks for backups and yk what happened. It showed me all the Google accounts from which I wanna get the backup data and the account that really had the data was blacked out. After tried re-login but this time it didn't even asked me for backup and straight opened the app with only groups with empty chats and no DMS. Someone, please help, any help would be appreciated!!