Diagnosis
You're likely hitting a limit of the built-in speed test server or there's a configuration bottleneck. The Alien's hardware can handle gigabit, but you need to check a few things.
How to Fix It
First, bypass the app's test. 1) Connect a computer directly to one of the Alien's LAN ports with a good Cat5e or Cat6 cable. 2) Run a speed test from a reputable site like speedtest.net or fast.com on that computer. If you get near gigabit there, the router is fine—the app's test server is just not accurate at the highest speeds. Second, in the AmpliFi app, go to Settings > Internet > Smart Queue Management (SQM). Make sure SQM is turned OFF. While great for preventing lag, it caps maximum throughput.
If That Didn't Work
Check your modem. Is it a DOCSIS 3.1 modem (for cable) or capable of true gigabit? An old modem will bottleneck you. Also, log into your modem's interface (if it has one) and see if it's reporting any uncorrectable errors on its line—that can cause speed caps.
Heads Up
WiFi speeds will always be less than wired, especially on the 2.4GHz band. For testing your internet pipe, a wired connection is the only way to get a true reading.