Slower speeds on router than on modem. Nighthawk AC1750 r6400v2

I just purchased an Arris surfboard8200 and a Nighthawk 1750 6400v2 and have them all set up through Suddenlink. I have a gigabit plan through them and would get 700-800 Mbps down on our old modem/router combo. On this new combo, I'm getting 60-100 Mbps for some reason. I went into the router settings and it says the ports are set to 1000M so they're fitted for gigabit, right? I switched my ethernet cable from my PC that was plugged into the router directly into the modem instead and I'm getting 800Mbps, but plugging it back into the router gives it that same 60-100 Mbps again. What gives? I updated the firmware just now to 1.0.4.122 and I tried disabling armor since I read that slows down your speeds drastically, but I'm still getting notifications from the nighthawk app that armor is scanning my devices. How do I truly turn it off? Is there anything else that could be limiting my speeds from the router? I used the basic yellow ethernet cable from the box to connect the router to the modem, could that be it?

Edit: I tested it, and on the yellow 5e cable that came with it it has the worst speed, but with a white cat7 cable that I used to use with my PC to plug it into a modem at my old place, we're now getting 200Mbps or so. Better, but still not nearly the speed we can get plugging it into the modem directly.