My iCloud backup stopped in the middle even though I have plenty of storage what should I do?

I received my new phone (iPhone 15 Pro max) and I began to attempt to sync everything but had trouble doing so. That said, I took it to my wireless carrier, who informed me that I needed to do an iCloud backup because my old phone (iPhone 13 pro max) had never been backed up, which I was unaware of. So I began doing a backup and it took about 1.5 days when I kept checking it and when I went to check it tonight (it was a little over halfway done), it said "iCloud backup could not be completed". I do not know what to do. After all, I know I have plenty of storage because I purchased 2 TB of it.