@Dimalvel 

Hello, may I know does your RE connect to router by Ethernet cable or by Wi-Fi? 

If you connect Modem --- RE rather than Router to RE. you may have this problem, as modem could only assign 1 IP address to a single device. 

May I know the model of your router and your network topology, what is the IP & gateway on the device which has Internet from RE? 

https://www.tp-link.com/support/faq/838/

@Dimalvel Hello, thank your update

Here is the latest firmware for TL-WA850RE and make sure firmware is the latest first (need select hardware version firest)

https://www.tp-link.com/support/download/tl-wa850re/#Firmware

May I know did you set up a different SSID for RE from router's , if devices connected to router at the time, is there any problem? 

For "If I disconnect my phone then I can connect my laptop etc etc" what is the Error message if you try to add the 3rd device? 

What are the model & OS of the Wi-Fi clients?Can you login to RE and share us a photo of the client list at that time? 

> Do not use static IP on your client, but let them obtain IP address automatically to avoid IP conflict. 

> Please check if there is any advanced settings on router, like access control, whitelist on router. 

>May I know the model number of your router and your network topology? Many thanks~