iCloud backup problem

You should try  a forced restart: 

Force restart iPhone - Apple Support

This is different from a simple Power Off. For me, the logo appears  5 or 10 seconds past the "slide to power off" message. Wait for the logo; don't do the slide thing. 

What is your storage on this iPhone? At Settings>General>iPhone Storage, my phone says 50.5 GB of 128 GB used. What does yours say? 

If the storage is nearly full, more than 90% used, then your phone may not be able to synchronize data. Many operations use the iPhone's empty storage space to move files around. If you run out of storage, then those operations may not be able to be completed and, ironically, that can mean that even deleting doesn't work. If you have less than 10% available, then things can get lost forever.

Also, at Settings>Battery, what do Battery Health, Maximum Capacity, and Cycle Count say?

Keep in mind that iCloud is designed primarily for mobile devices, and since syncing uses the battery, and since lots of people are very anxious about battery drain, the iPhone will avoid syncing when not plugged in. And since syncing uses processing power, and people hate slow-downs, the iPhone will avoid syncing while you are using it. And since syncing requires bandwidth, the iPhone will avoid syncing when the internet connection is weak.

So, if you want faster syncing, then find good wifi, plug the phone in, and leave it alone. All this applies to a lesser extent for the Mac.

Thank you for your help, but I’ve already tried those solutions—it seems like the problem is something else, and that’s causing me trouble. iCloud prices in Turkey are very expensive, and I can’t easily store videos or photos. By the way mine is 111,09 of 128GB