DIAGNOSIS Your devices are probably stuck on the 2.4GHz band, which is slower but has longer range, or a key feature like OFDMA is misconfigured. You're not tapping into the WiFi 6 speed.
FIX First, make sure your client device (phone, laptop) actually supports Wi-Fi 6 (802.11ax). If it does, ensure you're connected to the 5GHz network. Run a speed test with a device wired via Ethernet to one of the LAN ports. If that's fast, the issue is wireless. In the router admin, under Advanced > Wireless > Wireless Settings, make sure "802.11ax Mode" is enabled.
IF THAT DIDN'T WORK Log in and check for a firmware update (Advanced > System Tools > Firmware Upgrade). Also, try changing the 5GHz channel width to 80MHz (from 20/40/80 auto) and pick a less crowded channel, like 36 or 149, to reduce interference.
HEADS UP The AX1800 speed is a combined total for all devices. A single Wi-Fi 6 device will typically max out around 1200 Mbps on the 5GHz band in ideal conditions, so don't expect the full 1800 on one phone. Real-world is always less.