iPhone never automatically backed up to iCloud

Every 2 weeks my iPhone 12 mini tells me that the iPhone was not backed up for the last 2 weeks. I then make a manual backup (that not always is completed – sometimes I have to start more than once until it is completed) – and after two weeks again the same ...

Btw. I have an iPhone 12 mini with 256 GB and 50 GB in iCloud (among 25 GB used); for backup I am told that backup size is 6.6 GB. So space cannot be the problem. I do not have cable bound router, only mobile data (5G). I have an older second iPhone SE that also has mobile data and is sometimes used for hotspot – and then the 12 mini is connecting through the SE (even if the SE only has 4G and not 5G).

What's going wrong? Why is the backup never done automatically (even if all is configured to)?