Significantly lower speeds when using a router

[CouldBeCrazy]

So, admittedly, I am very new to networking. I've never had to set one up by myself, before. Every time I've moved in somewhere, it was already done for me. That being said, I've run into a major snag with my current setup. The hardware I'm attempting to network with is an Arris DG3270 modem/gateway that was included with the activation of my internet service, a Netgear AC1900 Nighthawk router, and all of my cables are 10/100/1000 speed rated.

My internet speed is 250mbps, and I readily get those speeds when I'm not using a router, and just plug directly into the back of the modem with an ethernet cable. I will NOT, however, get those speeds if plugged into the modem while the router is also plugged in. Even if i have no other devices connected to the network, at best i get 30-60mbps on my desktop, which is connected via ethernet cable (it is a cat6 cable). If I completely unplug the router, i can obtain 250 mbps again by plugging into the modem, otherwise, i'm stuck at a fraction of the speed i pay for.

Is there any way i can reach a higher bandwidth while using the router, at all?

I believe the gateway that the internet company provided me is most likely the problem. I don't know how to set it to bridge the connection to the other router.

 

[Elysian890]

have you configured the router manually?

if not, do it, assign a static IP, configure DNS and DHCP server, in most cases you'll have to disable modem DCHP and let the router assign IPs over the network
to configure everything you need to access setup via 192.168.1.1 or 192.168.0.1

if you connect the router to the modem and the computer to the router, do you have this problem?

 

[CouldBeCrazy]

if you connect the router to the modem and the computer to the router, do you have this problem?

Yes, I'm configured that way, currently. The PC connects directly to the router, which connects to the modem/gateway. I don't know much about configuring this stuff. I'm afraid I'm going to mess something up if i go in blind and start changing things.