Read some of these about ideas what to try to fix it:
Previously, I had an Asus AC3100 router and now I have a pfSense box.
Taking the CenturyLink modem out of the equation helped get both to full speed, as did make sure “cut through forwarding” was enabled on the Asus.
To get rid of the modem, most CenturyLink customers will have to enable VLAN tagging. Google “CenturyLink fiber VLAN tagging” and you’ll find plenty of information.
Good luck. We’ve been super happy with our CenturyLink fiber service.”