Thanks for the reply :)
I had only the Remote Access Config enabled from Administration > System that did allow the app to work remotely with DDNS but I think this is more passive so only updates when the app is connected to the AX8 and updates the latest WAN settings so if the IP address changes in the meantime it doesn’t update the app when it try’s to connect remotely.
I have now turned on the DDNS Client in WAN > DDNS and switched off the FFTP ONT to generate a new IP address and the app did reconfigure over 4G to connect. The guidance on this page though is somewhat confusing or I am reading it wrong; 
‘The host name is successfully registered. You can use “[hostname].asuscomm.com” to access the service in home network from WAN. Use “[hostname].asuscomm.com” to remotely access your network.Go to Advanced Settings > WAN to configure the port forwarding or DMZ settings to allow other WAN clients to remotely access your network. If you want to remotely configure the wireless router, go to here.’
As I only want to configure remotely the router I’m not sure if I need the DDNS setting on or off as the remotely configure the wireless link mentioned above takes you back to  Administration > System > Remote Access Config which is what I only had originally on.
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