Slow speed through 3rd party router for gaming

First, thanks for any assistance anyone can give. I'm using an Arris BGW210-700 Gateway for 1gig fiber service from ATT.

As Arris doesn't have Port forwarding for multiple devices (to open NAT settings for online gaming) and I have 3 gaming units (2 Xbox Ones and a PS4) I have set up my Linksys ea6900 wi-fi router and port forwarded the channels I need for my NAT settings to my router through the device list in my Arris Gateway and those devices are the only ones on the network for that wi-fi router. My issue is that the mbps speed has been decimated. I have a CAT7 cable connecting the gigabit ports on the gateway and the router but the max mbps i can get on a device is 25mbps. Through the gateway my speeds max out at about 180mbps on my wireless N connections. I have tried both bands on the Linksys router, as the gaming consoles have wireless N integrated so I'm only looking for 100-150 mbps and I should be able to get that on either band, right? I've tried to set up the channels to optimize speeds, but it hasn't helped. I know the service is recognizing the gigabit port, as it states in the Arris device home network status screen that the connection is running at 1gig. I am unsure what to do from here.