Weird issue with internet speed with wifi enabled

I've been in a constant battle with my isp regarding my internet speeds until I purchased my own modem to look into the issues myself. After fooling around with all the settings, disabling the router mode on the device speeds caused 5mbps speeds to increase to 800mbps. Turning router mode back on causes it to drop again. The router supplied by them was also a modem combo. What exactly could be causing this to happen? I need wifi for every device in my home except my computer otherwise this would be an easy fix.