Depends on the layout of the gym more, to be honest. Are there many walls? Are the walls reinforced concrete (do they have iron in them as that kills the signal quite a bit). If you can post an image of the approximate size and layout of the gym, and positioning of the router, would be easier. But best case would be having the router somewhere in the middle of the gym, at a higher point (not on the ground) and it should reach anywhere within 50 meters radius of the gym which should be enough as you said the gym is not very large. Also when it comes for a router like this - general usage router, people browsing facebook on their phones while training, youtube, etc, you don't need a dual-band router.