Router speed problem

Hello,

So I have Rogers Hitron 4582 (Giganet) and the AC 86U. I did a speedtest (fast.com) and got about 350 mbps. I reset my modem ran straight cable speed test off modem and got 1.2 gbps. Hooked back up to the router and speed test drops down to the same 350 mbps. Modem has been switched to bridge mode.

What's up with that???

Thank you

Mike