I'm really struggling to understand / fix what's happening with our home network. We are (2) Deco W7200. We have a 300 Mbps / 300 Mbps plan with Verizon FiOS. We do not use a Verizon router so there is nothing else between the Deco and the ONT. IoT and guest networks are disabled. QOS is disabled. Fast roaming and Beamforming are disabled. Firmware is current on both devices.

This setup has worked just fine for quite a while but, a week or so ago, I started noticing problems. Whenever we've had issues in the past, they turned out to be Verizon issues. So, I had Verizon come out and check their equipment. That's where it got more interesting but here's a rundown on all the steps I've taken:

1. Confirmed that nothing changed with the physical surroundings, radio interference, cabling and/or configuration. -- No issues found.

2. Shut down and removed the second W7200 just in case that was creating a problem. -- No change.

3. Made the secondary W7200 the main router (while leaving the original disconnected). -- No change.

4. Verizon tech plugged into the ONT directly via ethernet... that was behaving correctly and getting right around 300 Mbps.

5. There's a Cat5E cable running from the ONT in the basement to the router on the 1st floor. The tech connected to the ONT directly using that cable. Still getting around 300 Mbps.

6. The tech reconnected the router to that same cable then plugged his device into the second port on the Deco. We weren't able to get any more than about 124 - 150 Mbps out of it.

7. Tried moving the Decos to several different spots and re-testing. -- No significant changes.

The speeds reported by the Deco app on my iPhone and iPad are all over the place. One second 23 kbps / 25 kbps. Less than a second later, it'll jump to something like 295 kbps / 600 kbps. Up down, up down. It will report anything between 0 kbps and 135 Mbps.

I'm at a loss. It seems very unlikely that the hardware in both Decos would fail at the same time and the cabling checked out fine. Could this be a configuration issue?

Any help would be VERY appreciated!