How to top up a new PAYG electricity smart meter? No instructions left after installation!

Hi, I had an electricity smart meter installed yesterday, and was left with no instructions on how to top it up (I’m on a PAYG tarif). Can someone explain? I downloaded the app, no payment or top up options are shown. Online, it still thinks I have a traditional meter and doesn’t allow me to top up. 

I’ve been trying to call OVO but their phone lines just hang up, and have tried online chat several times during their opening hours and it just says “chat is unavailable”. There’s just no way to get in contact with anyone. What an awful experience.