I bought my iPhone 8 plus on 23/09/17 when I was in Seattle and have been on a cruise for 70 days I have taken all together 4500 photos and videos and have only just got home to decent Wifi. I have been trying to backup my phone 48 hrs

I bought my iPhone 8plus on 23 September 2017 and have been on a cruise where I have not been able to back up my phone contents. Now that I am back home I have tried to back up my phone to my 200 GB cloud and it has been plugged in and connected to the Wi-Fi for 48 hours and it has still not backed up. I have taken about 4000 photos and about 150 small videos. How can I back up My telephone it seems to be trying to back up but it will not continue and finish