@bennys Hi Bennys, 

Sorry for the late update. 

1>Please verfify your Wi-Fi password by forgeting your Wifi 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 : press the reset hole for 10 secs with RE powered on

On cell phone WIFI settings, connect to Range extender WiFi first ( do not connect to router wifi or and turn off mobile data)

3> follow instructions on Tether app 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 )
 

Here are some set up videos that may help:

 https://www.tp-link.com/support/download/re505x/

@bennys Hello, 

Do you mean RE's Internet will work for few mins or hours, WiFi LED lights up after installation but Internet drops again ?

May I know how about the Led Status on RE505X when Internet drops?

Please make sure RE505X firmware is up t odate and here is the latest firmware: 

https://www.tp-link.com/support/download/re505x/#Firmware

If RE505X WiFi LEDs are still ON when Internet drops, please turn on the DHCP on range extender like the photo attached:

If RE505X Leds are Off  when Internet drops, put RE very close to main router, and set up a different SSID for RE and see if issue persists.

If still does not help, may I know the Model number of your router? And have a photo of your settings on range extender, thank you~

@bennys Hi Benny, 

You could factory reset RE and login into its Web page to configure RE 's DHCP settings, here is a more detailed instruction to explain how to do that. 

After that, run Quick Set up for range extender and connect RE to Wi-Fi router

What's the solution if my TP-Link range extender can't connect to my router properly? (RE Led On but No Internet)

https://www.tp-link.com/support/faq/2965/

At the same time, we'd like to follow up your case via email with subject 【Forum ID 257332】 in case you need any further help.Please check your inbox later~