Are you getting those speeds on the 2.4 or 5 ghz band?
Assuming your router has no issues (and firmware up to date), no nearby devices with interference and your cables are sound (try different cables, ideally cat 5e or Cat6) from ONT to Router. Perhaps try the following one at a time and note the speed test after each item (i usually use 2 different speed test checks) till you fault find.
1. Place your antenna upwards or horizonal
2. Reboot router.
3. Disconnect all other devices connecting to your wifi and only connect 1 device to it (kill all background programs). If this sorts it out, its likely you have something in the background eating up your bandwidth.
4. Try changing the wifi channel (try 1, 6 and 11).