I have been having this same problem on my wifes iCloud backup for iOS since her 12 Pro Max, now 15 Pro Max.
it just won’t backup, no matter what I so. My iPhones iCloud backup works 100% every evening. No problem.
my eifes just sots there after 2 days saying 48hours remaining the. Just fails after a few days. Same WiFi, same process as on my iPhone and nothing works.
we need Apple to expose some details on WHY the backup fails that we can try fix the data we try to backup if that is the issue.
I went to the iStore in South Africa, many of them, they can’t help us either???