which method (extender or repeat mode) works better? Assuming the additional router would need to use wifi as a connection to main router(bc the main is permanently installed in a back closet wall and the isp did not provide any additional slack when installing fiber into the house-restricting relocation to a more open area in house)? Can the 2nd router be connected via ethernet (basically side by side in closet ) and still broadcast a better/stronger signal or cover a further distance? I’m trying to get a signal to my garage that is detached, probably 150ft away from main router.
Your optical cable should terminate in your house in an ONT (Optical Network Terminal) Normally a box, but sometimes a box inside a metal case. This is what should decode your optical signal into one a router/modem can use. The modem should be plugged into that box with an ethernet cable and one you can unplug from the ONT and exchange for a much longer one that will allow you some options to relocate the router/modem. 
Spectrum. The ONT box is the modem. The modem is the one stuck inside the wall panel- with enough space to put a router next to it. I realize I could run long cat 6 and extend that router maybe on other side of closet- but any longer would be a mess(renter) - then running another cat 6 from main router to 2nd router- complete mess. So if depending on wireless connectivity between 1st and 2nd router- extender mode or repeater mode? Thanks
That’s a shame darn , cause running a cat 6 say 50ft long to a modem/router would be so easy. I am a tad confused as an ont is just an endpoint for the optical service and not a router, well from what I know at least, and in my case houses the plugs for the ethernet cable to the modem/router, so now I have to do some more reading lol. Always more to learn.
The other best way is using a powerline setup.
The main problem would be the router placed in a confined space. That blocks WiFi signals. I think Ron has possibly the best solution for you, especially if you can trace your circuit and find outlets on the same one as the power for your router. 2.4Ghz will be the best choice, for signal strength, exiting the closet, depending on the composition of the walls.