@Deano22  Hello, 

' No Host network connection' error could happen if wrong wifi password was accidentally  put in during set up,  please try suggestions here: 

1>Please verify your Wi-Fi password by forgetting your Wi-Fi network on cell phone first :  
Go to Settings-> WiFi on your phone, Tap (or Long press  the router's WiFi name  ->Forget / Delete the network 
Please reconnect to your router wifi and it will ask for a password, will it accept your router password?

2> factory reset tp-link range extender and follow instructions on Tether app or Web page step by step  ( carefully examine the Wi-Fi password you have put in, and it is recommended to create a different SSID/WiFi name for RE )
RESET:  with RE powered on, use a pin to push the reset hole until all the Leds flash once.

3> confirm your range extender is on the latest firmware : https://www.tp-link.com/en/support/download/re200/v4/

4> For Dual Band Range Extender, if the App says "No Host Connection", it's probably because the router's 5G is using the DFS channel,  it is recommended to disable Band Steering on the router as the guide below and, change 5G channel to Band 1 Channel 36, 40, 44, 48:
https://www.tp-link.com/en/support/faq/3143/

If the concern persists despite the above steps, can you help confirm the details here to locate the issue:  1. Led status on Range extender after you set up the extender / 2. model of router/3.   do you connect Range extender 2.4G or 5Ghz band? Thank you.