Videos won’t play?

Try double clicking the Home button and swipe Safari upwards. Go to Settings/Safari and clear History and Website Data. Open Safari and test.

Safari - Clear the history and cookies on your iPhone, iPad, or iPod touch

Safari website data on your iPhone or iPad - How to clear

Try a forced restart.

Reset Device

Reset – iPhone 7

Hard reset

Clicking the Play button at the center doesn't make the video play?

Is there any network speed throttling in effect?

Does it play when you move device closer to the router?

Try changing the location of your device until you see 3 or 4 bars instead of 2.

.

Do you have access to another network where you can try to log in?

Have you tried restarting/resetting your router?

This is little bit extreme.

Backup, set it up again, and restore.

Test.

How to back up your iPhone, iPad, and iPod touch

https://support.apple.com/en-us/HT203977

Put your device in recovery mode and set it up again

https://support.apple.com/en-us/HT201263

When I have this kind of a problem with my iPhone 6, usually a restart helps.

I am sure you have tried that.

https://support.apple.com/en-us/HT201559