Hello everyone

This problem is related to the 5Ghz channel on Airtel router. 
Tp-link Range Extenders (EU) version supports : Band1/2/3 on 5Ghz to accord with the results of the CE certification as listed below: 

RE (EU) version 5Ghz Band :

V Band1:36-48   (36,40,44,48 )；
V Band2:52-64   (DFS) ; 
V Band3:100-140 (DFS);
X Band4:149-165 (149,153,158,161,165);

So Airtel router's 5Ghz Wi-Fi channel should on Band 1/2/3 or RE(EU) will not able to detect its 5Ghz. Here are some suggestions for  this issue: 

1. Please contact Airtel support check what is current 5Ghz channel on Airtel router. 
2. Change Airtel router Wi-Fi channel to Band1 Channel 36/40/44/48 if possible. 
(Band 1 is recommended  as Band1 is more stable compared with Band2/3 )
3. Try several reboots on Airtel router until router reverts to a Band 1-3 channel. 

If the issue persists, please start a new thread here with the information below to look into the concern the further. 

1- model and hardware version & country version (EU/ US/ UK etc.)of your device

2- model of router

3- photo of router's 5Ghz wireless settings  
 

@Aws_new 

Kindly refer to this thread and change router' s 5Ghz channel to 36/40/44/48 with the help from router's support. 

[Troubleshooting] TP-Link Range Extender is not detecting the 5GHz Signal of the Main Router
https://community.tp-link.com/en/home/forum/topic/265818

It is recommended to report back with information in the above thread so we could look into the issue further. 

1. model of your  router  and share with a photo of router's WiFi Channel settings.

2.RE's model,  hardware version and country version (EU/US etc.) on its back label, thank you