Question Internet speed drops after download or speed test

Oct 20, 2020
When I use just my modem, I get my ISP's advertised speeds with no problems. I can download at 30 megabytes a second all day.

When I plug my modem into the router, everything is fine until I start a download. After a minute or so, the speed goes from 30 megabytes/second to about 30 kilobytes/second. Ping time to goes from 20ms to 1-3s and I start dropping packets. This is persists even after the download or speed test is done until I reboot the modem and affects all devices on the network, both wired and wireless.

I've tried factory resetting the modem+router, updating the router's firmware, getting a new (different kind) router, using new ethernet cables.

Router is a TP-Link AC1750
Modem is SB6183