First of all, this is the Apple support community, so if you are looking out for help from an Apple [support] employee, I might suggest to you to reach out at Contact - Official Apple Support, because if you have followed all (or most) troubleshooting steps and don’t see an improvement, I am unfortunately unsure if we may be able to help you.
Anyways, just to be sure, important steps to note:
— ensure a stable internet connection (as noted above) with low data mode turned off
— ensure your iPhone is charged (as noted above)
— ensure you have not paused iCloud synchronization, or try to manually sync using the icon in the very top-right corner in the photos application (the picture associated with your Apple account), it possibly even tells you the reason why syncing might have failed
— ensure that syncing your respective devices is even turned on - settings, [your name], iCloud, Photos (under “saved to iCloud”), make sure sync this [device] is toggled
I am currently looking into the matter for more information, if you have something discovered in the meantime or if your issue resolves, feel free to tell us to potentially help others with the same problem.
edit: of course check your iCloud storage space as well; as Ingo2711 suggested