Hi everyone, first post here.

I purchased a 3-unit M9 Plus from Costco in August.

Initial set up was surprisingly a pain in the neck, but eventually successful. It had been working fine for a couple of months but recently I noticed a significant decrease in speed. 

After some tests it turns out that the 5Ghz network is missing. It is technically enabled in the app but it is non-existent. Tried disable/enable in app a few times, no change. Tried unplugging the whole system a couple times, no success. Wifi Analizer app can't find the 5Ghz either.

Then I factory reset the whole system. When I set up the first, main Deco (the one that receives internet from the modem via cable), there is proper 5Ghz and achieves max speed. But once I add a second or even a third unit, 5Ghz disappears completely. During one of the tries I seem to have maintained 5Ghz with 2 units but that happened only once and after adding the 3rd unit, 5Ghz disappeared.

I tried swapping the units and making a different one the main unit, same story. First, single unit does have 5Ghz, but once I add 1 or 2 more, no 5Ghz, only 2.4Ghz.

I tried this reset/set-up procedure 4-5 times, no 5Ghz. System states it's up-to date, no updates needed. 

The app's own internal speed test however shows max speed, using the hidden(?) 5Ghz network?? No other device has 5Ghz and speed test is slow, even when placing the test device right next to the main Deco.