How to setup a booster for a Xfinity box (xFi?) internet?

Thanks for taking the time to write a response. Yes, I did that and the very first instruction was to login as admin and set the router as a booster. However, to do that I need to disconnect the xFi box and replace it with a modem, then connect the tp-link as the main router. Does that make sense? So I was hoping to learn the easy way of doing this (without replacing etc etc). Any thoughts? 

@user_74yum8​  Modes they usually can run in, if supported in firmware.

Normal stand alone router mode with modem and/or Gateway in bridge mode (gateway set just as a pass through device)  -- what you saw.

AP mode.  Router as a WiFi Access Point.  You'd run Ethernet cat 6 cable to your gateway.  Devices could connect either to your gateway or the Access Point.  Sometimes can use the same SSID WiFi name, other times it would have to be unique.  The manual could tell you.

Repeater mode (If it has it).  You'd search for your Gateway's WiFi signal with the TP-Link, then connect to your gateway on the TP-Link.  It would then rebroadcast your gateway's SSID WiFi name.  It's slower because it's intercepting signal and re-transmitting it.  If you get it too far away from your gateway, it won't work well at all or be really slow, and may slow all of your WiFi devices down because you would be flooded with errors. 

Thanks so much, @flatlander3 for all the details. Also, I never thought about "If you get it too far away from your gateway, it won't work well at all or be really slow, and may slow all of your WiFi devices down because you would be flooded with errors. ", very good point. Thanks again!