iCloud photos not syncing on iPhone

I can't recall which iOS upgrade caused this, but I've been having the same problem for months. I have to manually ask it to sync from within the Photos app every time I remember, which is time-consuming and tedious, given I'm used to it automatically syncing when on Wifi - and if I'm working I don't have time to wait.

Phone is connected to both to wifi and internet, good signal, super fast. iCloud is set up synchronise, low power and low data modes are both off, plenty of room in iCloud, phone even plugged into charger.

I noticed it isn't automatically backing up the whole phone to iCloud either, although the device is set to do so.

Any other ideas please?

Is your device connected to the internet?

If you connect to Wifi, doesn't necessarily mean you are connected to the internet. --> Internet not available WiFi and Cellular … - Apple Community

Wi-Fi Assist is on by default. This will automatically switch to Cellular Data if the device is not connected to the internet.

About Wi-Fi Assist - Apple Support

The Photos app and iCloud Photos sync.

Enabling iCloud for Photos as depicted in the provided picture, means that if you delete any or all data from one device, it will also be removed from iCloud and all other devices linked to the same Apple ID. Likewise, the reverse is true as well.