Try unplugging your router for a few minutes.
You could also try your phone data just to make sure it’s not the internet.
Try using your phone as a hot spot.
If it still doesn’t work then try setting the stick back to factory settings. Although that’s a worst case scenario!
Router reset, stick been set back to factory settings.
Can’t think of anything else.
@Claire_39 did you try your phone as a hot spot? Sorry can’t recall.
If I’m not mistaken should be the same as connecting to Wi-fi?
When you say after 5 seconds it turns off, is that the television screen goes blank and the white light on the Stick disappears ?
Or does the white light on the Stick starts to flash ?
If the white light on the Stick disappears, then are you using the supplied power adapter with the Stick or using a USB 900ma port on your television to power the Stick.
Try both if you can and if you have a spare USB micro to USB standard cable plus a USB 1A or higher charger try that has well.
I don't have mobile data on my smartphone, but would have thought go into your phone settings and activate it's Hotspot feature, then go into the Stick Settings under Network, setup new wifi connection and see if it shows your smartphone on the Stick Network list to choose from.