@Paddy99 

Hi,

Accessing the extender via tplinkrepeater.net might not work under certain circumstances. In such a case you need to access the extender via its actual IP address.

If you have not manually set a LAN IP address on the TL-WPA855RE it will have been assigned an IP address by your router. To find out what the current IP address of the TL-WPA855RE is you can log into your router and look it up in the router's client list or run some network scanner software on a computer that is connected to your network.

  @woozle yeah it's for a static IP but I can't ping it or access it through the Tether app after changing it to an AP although it does appear to be working 

  @Paddy99 

If you mean you have set a static IP address within the "LAN Settings" of the TL-WA855RE, then make sure that IP address falls within the subnet of the router's network.

In case the extender had a static IP address outside of the subnet of the router, then that would also lead to the symptoms you described.
 

  @woozle yep all in the same range. Just almost that when you change it to AP mode it disables ping