I'm confused on how to remove the demo mode

That doesn't sound like the behavior of a phone in "retail Mode" to me... I could be wrong so to turn it off (if it IS on) Go to 'Settings' typically this is a gear shaped icon in your pull down menu or home page... Scroll down if necessary, find and select 'Display'.

Next select 'Advanced'

Find 'Retail mode' and select it...

select "exit retail demo mode"

Now enter code 24682468 to confirm. (Hopefully)

If it wasn't on you can try a soft reset... turn it off, not just pushing the power button for standby.

That fixes many issues, yes, a simple restart...

If that doesn't work try a "Factory Reset":

"Settings"...

"System"...

"Reset options"...

"Erase all Data"...

This will perform a 'Factory Reset' returning it to an out-of-the-box state.

All your Data will be gone and you will have to set everything up again...

IF YOU are SURE you want to do that "Confirm" and wait for it to restart.

Good Luck