Oh, I see and I very much understand the problems one gets with Double NAT.
Before we begin, make sure that Airtel Modem settings are untouched. (Let PPPoE be handled by Airtel modem for the purpose of simplicity)
The method is as follows
1. Log into your
Asus Router , Advanced Settings -> LAN , Input the following
IP address : 192.168.0.170
Subnet
Mask : 255.255.255.0
2. Also click on DHCP Server > Disable it (on your Asus Router)
3. This is something you need to do manually.
Make sure the following Ethernet cable connection.
Airtel Modem (Port) --------- Ethernet Cable ----------- Asus Router (LAN Port)
I.e. in the LAN Port of your Asus Router , not the WAN Port.
After that,
Connect to your Asus Router's WiFi and run a traceroute on your
Laptop. Also Turn OFF Airtel Modem's Wifi.
You would see a single NAT only , i.e. of your Airtel Modem.
That's all, it's done!
All of your devices will get IP's from the DHCP of your Airtel Modem.
The reason I suggest this method because setting up PPPoE on your Router can be complicated with Different Modems.
This way we are eliminating Double NAT by removing NAT from the Router which is easier and the PPPoE and DHCP continues to be handled by the Airtel Modem just for simplicity purposes.
Also please do tell about how DDNS is now working.