I’m puzzled. Using my cellphone (AKA mobile phone) connected to internet through 5G data I can see the files and folders in Files > iCloud Drive but can’t download them for viewing. The download sticks permanently on ‘waiting’. Connecting through WiFi there’s no problem.
There is also no problem with Mail or Safari, for example, via 5G data.
In Settings > Cellular/mobile data I have both WiFi assist and iCloud Drive turned on.
iPhone 13 Pro, iOS 15.5
I just had that happen, except it was LTE rather than 5G. I looked at Settings > Cellular (I’m typing that from memory, so it might be slightly off) and Files was enabled, but the document would not download.