Diagnosis
Your device is likely connected to the 2.4 GHz band instead of the faster 5 GHz band. The eero 6 combines both bands under one network name (SSID).
How to Fix It
You cannot manually band-steer on the eero 6. The system decides. To encourage a 5 GHz connection, temporarily disable the 2.4 GHz band. In the eero app, go to Settings > Troubleshooting > My device won't connect. Select the device having issues. Tap 'Temporarily pause 5 GHz'. Wait 10 seconds, then tap 'Resume'. This forces a reconnection and often pushes the device to 5 GHz.
If That Didn't Work
Legacy mode may be enabled. In the app, go to Settings > Network Settings > DNS. If it's set to 'Automatic (from ISP)', change it to a public DNS like Google (8.8.8.8). Also, under Settings > Discover > eero Labs, ensure 'Band Steering' and 'Optimize for Conferencing and Gaming' are ON.
Heads Up
Speed tests over WiFi are variable. For a true test, connect a laptop directly to the gateway eero's LAN port with an ethernet cable and run a speed test. That isolates the WiFi as the variable.