I have a Tecno spark 8c and it can't download attachments using mobile data from the Gmail app. Works when I'm on wifi. Must be some app like the Download manager having an error or some metered connection thing(I did check for that for apps like Download Manager n Downloads).
Workaround for gmail me was logging on gmail in chrome. K-9 mail also works. So it had workarounds.
Recently it started affecting Tachiyomi downloads too. Can't really ignore it now when I'm not connected to wifi. Did check for it, but didn't find a reason/solution. Been thinking about a factory reset, but not really keen on it as I would have to backup files(important ones do have copies) n app settings.