@cedjrdfire From your description it really looks like the DoS Protection was engaged. Turn it off when you're on site and test.

@PeteEvoWireless Have you tried to tweak the MTU Size (lower it down) when configured with PPPoE connection ? If you haven't try that and see if any difference.

@JennyColl Here's a simple router selector from TP-Link you could use to select a device, which meets your requirements. When you're ready with your choice you may ask the community if it's reliable...

@Whitebunnie It's best to contact your ISP, explain what hardware you have (TP link AC1750, which is not a broadband modem/router) and they'd tell you if you need any additional hardware and settings...

@ZSON The idea of the solution is before building WDS, to set the host device (Airtel GPON Router) on U-NII-1 band (choose one of these channels 36/40/44/48) reboot both devices, then build WDS. When...

@ZSON You may test ths solution and check if it helps.