Ok, I've been having this issue even with a brand new modem and router.
Wired straight to the modem I'm getting 150-200 mbps consistently.
Now on my dual band router I'm getting anywhere from 5-50 inconsistently. It goes up and down all day and this is on the 5gz band standing right net to it. Only Getting about 6mbps wired to Router.
Uploads are abou 6-7 wired to modem or on WiFi.
Asus RT-AC1200 V2
FYI, I'm pretty un-knowledgable with networks and settings