Download speeds slow down suddenly when plugged into router

[aethlive]

I've been having this problem for a couple days and even bought a new router to try to fix it. My previous router was a Netgear N600. My internet speeds are fine when my laptop is plugged straight into the modem(ARRIS SB6121), 95 down 18 up (i pay for 115 down 20 up). Then once I plug my router(Nighthawk X4S which is brand new) into the modem the download speeds slows down, 3 down and 17 up.This is a really weird problem, when I connect my laptop to the modem and run a speedtest everything is fine but as soon as I plug my router into the modem it goes bad, and its only the download speed. I have tried different ethernet cables, factory reset both the routers and im still having problems. The new router was working fine when I first set it up but a couple hours in it started having problems again. Any help would be appreciated thanks.

 

[corroded]

Unplug the Modem completely... ALL wires..

Leave it for 15 minutes. Plug it back in, then once it is UP, Plug in the new router.

MAC address thing on cable modems.

If it persists, call the ISP.

 

[aethlive]

Unplug the Modem completely... ALL wires..

Leave it for 15 minutes. Plug it back in, then once it is UP, Plug in the new router.

MAC address thing on cable modems.

If it persists, call the ISP.

I forgot to add that I have already called my isp about this and they scheduled a technician to come over but its going to take till next monday and i would rather not go 4 days with only 3mbps down so im trying all the options. I will try your method right now and update you when I finish

Edit: I tried this and it did not fix the problem