iPhone complains about "Poor network connection" but I'm sitting right next to the router

My network connection may appear poor, two out of four bars on Wi-Fi, but it is actually strong and I'm sitting right next to the router. I can stream videos and browse website without any issues. The problems are that Photos refuses to automatically sync to iCloud ("Syncing with iCloud paused") due to "Poor network connection", and iCloud Backup also doesn't happen automatically even though it is set up to.

How can I persuade iOS that my network connection is OK? Alternatively, how can I force iCloud backups and iOS Photos to sync to iCloud despite the allegedly poor network connection?

I'm using iPhone 12 running iOS 18, but had same problem on previous iOS versions.