We have seen some minor configuration issues. Remember having 2 routers on the same line, each one is trying to do the same thing. This introduces a double NAT or translation condition. On one side of a router is the Public IP address that comes from an ISP. This goes thru the router and turns into a Private IP address, i.e. 192.168. Along with other traffic and information. Putting another router online causes some of the traffic data to be misinterpreted causing some applications, XBL, or data to not be routed correctly. Most of this can be fixed by having the 1st or host router in the line to be bridged, effectively turning off it's internal router functions and passing along all data and public IP address information to the 2nd router. We've seen this mostly in DSL modem systems. If this can't be done, if the 1st routers has a DMZ option, putting the 2nd routers IP address it gets on the WAN side from the 1st router into the DMZ of the 1st router will resolve the double NAT issue in most cases. Seen this in some Arris cable modem systems. Barf.
Most Fiber systems I believe can pass along a public IP address to a external router. I would talk to your ISP and ask them about this and the support of an additional router on the line. I presume that since you have one already, it should be ok.
One thing you can do to check what you have now, look at the 2nd routers web page and look at the WAN side status. Look at the WAN IP address, does it show a 192.168 or a ##.##.###.###?
Over all, you'll need this router online, if you have other devices online while your gaming, this is the router that will help manage that and keep LAG down to a minimum