@BoneyM 

Hi, welcome to the community.

"Can I use one single mesh router from my already installed TP-Link M4 router to extend the WiFi range without connecting it to the new router?"

It is a little confusing about what you want to achieve.

If you are asking whether different M4 units can be used with separate ISPs, the answer is yes, such as:

ISP#1---Deco M4#1---<WiFi>----Deco M4#2                     Mesh SSID#1

ISP#2---Deco M4#3                                                           Mesh SSID#2

These two Mesh networks will be separate from each other.

How to Create a New Network in the Deco App

Best regards.