You'll need to make sure your WiFI is 2.4GHz (the Home Mini won't work with 5GHz). You can then update any changes to your WiFi within the app:
Go to the ‘Account & Settings’ page (you can access it by clicking on the icon in the top left hand corner of the app).
Tap on 'Devices'
Tap 'Octopus Home Mini'
Tap 'Update Wi-Fi Connection' and follow the instructions on the app