Cannot play or download a video on my iPhone?

cosmicdenial Said:

"Cannot play or download a video on my iPhone?: I haven’t got heaps of room on my phone, so I have to have optimise storage ticked - download originals is sadly not an option for me. I cannot download certain videos to save to files, send to friends, or even watch - some such as this one are simply 3 seconds long and won’t load. It’s not an internet issue. How else can I see these videos? They’re super important to me."

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Try Clearing Space:

I'm not so certain it is the files giving you all the issues. Many system resources may be being used by unnecessary items. So, consider clearing space, by performing the following...

A. Clean TV and Music Libraries:

Remove all unneeded files from your TV and Music Libraries. Full-Movies take up 2-4GB typically, and a bunch of those would take up a load of storage, ideally. So, if you have any movies that are not needed, then delete them.

Also..

B. Clear Temporary Internet Files:

Clear your Temporary Internet Files(Cookies & Cache) of Safari:

• Cookies perform remembrance of certain site-entered data.
• Cache keeps a snapshot of the last time you visited a site.
• Go Here: Clear the History and Cookies from Safari on your iPhone, iPad, or iPod Touch - Apple Support

Also..

C. Clear "Other":

Clear the storages named "Other". It basically contains unnecessary files such as caches that are used for system ease of use - all of which gather over time: How to Clear "Other" in your iPhone's Storage - Use Backup 

cosmicdenial Said:

"Cannot play or download a video on my iPhone?: I’ve purchased additional iCloud storage and changed the settings to ‘download originals’. How long do you think it will take to download my videos? There’s quite a lot of them and I can’t watch most of them at this stage :/"

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Time Varies:

How long depends on the speed(Data Transfer Rate) of your router, and that of you iPhone. Being GBs of data, it may take quite a bit.