@SaW61,

Ensure that the Enable Private Addressing setting in your phone's settings is disabled as this will conflict with some security settings.

Next, forget the network in the list of saved networks on your phone, as this can sometimes store incorrect information about the connection.

Double and Triple  Check that there is no VPN or Adblocking Application running on your phone, A VPN can be identified by a Key Symbol in your Status Bar,

If you need to configure the network settings and cannot access the app, the Deco nodes can now be connected and controlled via a Web Interface, try logging into your router through the web to see if any settings have been changed.

Last easy potential solution would be to go into the router settings and disable fast roaming, as IPhones sometimes have difficult switching between network bands and can get stuck on a node or band.

If you are still unable to connect to the internet, try to change the method of encryption that is being used for the Wi-Fi connection, as some devices are compatible with certain encryption methods.

If you never disabled remote management on your router, you should be able to still configure and access the router in the deco app without being on the local network. If this is no longer working, your router may need to be reconnected to your TP-Link ID for some reason. 

If you dont have too many custom settings, a router reset might just be the end all be all fix