I guess the router is included in the plan by default. They may not reduce any charges if you use your own router.
If you want to use any high-end router of your own, you may need to use it as a repeater.
I am on VDSL and not on fiber. Hence, people using fiber can provide a better answer
Pl understand that the instrument supplied by Airtel is GPon from which the output is an optical cable and not a lan cable. One cannot by pass it. Of course, you can attach your own router to it later.