Phone not recognising sim card. Window pops up which says 'insert sim card or to view a demo of this phone first press Demo"

Sim contacts may be dirty, either on the card or phone.
remove sim card and GENTLY rub a pencil eraser on the contacts to clean, then test. If you can access the contacts on the phone, do likewise. If not, then you'll have to use an Ether based cleaner, Like electrical parts spray ( available from your local auto parts store) .
be careful not to spray too much into the phone, as once the spray gets inside it takes longer to evaporate. You can counter the time by using compressed air to speed drying of the phone interior without taking it apart.
Hope this helps!!