iCloud backup is backing up the wrong device

My husband's phone has been messing up so I gave him my old one. His phone was non responsive so I couldn't back it up & it hadn't been backed up in 142 weeks 😐 So I put his sim card in the new phone & logged him into his account like it was a new phone. Now his old phone is temporarily working so I'm trying to back it up so that I can get his info on the new phone. The issue is, I'm hitting back up on the old phone, it says it's done, but then when I log in to iCloud online it's only backing up the 7 photos on his new phone. It's not doing any of the 2,000+ from his old phone. Even though that's the phone I'm clicking back up on. Anyone know what to do? I signed out & restored the new phone to see if that would help but it's doing the same thing.