I just received my new C4000 from COSTCO last week.  I will say it provides much better coverage in my house than my old router and the price was really good at $129.99. The 2.4GHz seems to be working really well. My doorbell camera no longer drops out.  The 5GHz-2 is giving a really strong signal in places where I barely had 5GHz signals with my previous router.  Now for my problem.  The 5GHz-1 is unusable.  It constantly drops off-line.  I have been watching it with WiFi Analyzer and one minute it’s there and the next minute it’s gone.  When it’s there long enough to connect to it, I only receive a fraction of my internet speed.  And by fraction, I mean very little speed, between .1Mb and 20Mb on any speed test.  My internet is 150Mbs down and 10Mbs up.  On the 5GHz-2 I usually receive around 150Mbs.  I have these issues with my iPhone, iPad, Roku, printer and both my personnel and work PCs.  The issue is there even when I’m in the same room as the router, no more than 3 to 5 feet away from it.

Here is what I have tried to fix the situation.

1. My router is in a clear location with nothing near it.
2. I tried all of the channel options for the 5GHz-2 - Auto, 36, 40, 44 and 48.  When it’s in auto it seems to select 48 so I manually set it to 48.
3. I turned on then off MU-MIMO and Airtime fairness.
4. Next, I did a factory reset and left the setting at their defaults.  This didn’t fix it.  It’s still dropping out.

I only have around 25 devices connected to the router: 4 wired and an even split between 2.4GHz and 5GHz.

Other settings:

1. I have a few DHCP address reservations
2. I use Cloudflare DNS servers 1.1.1.1 and 1.0.0.1.
3. 5GHz-2 is using Channel 153 (works great and is the same channel I used on my old router).
4. 2.4GHz is using Channel 1 and works great.
5. Firmware 1.0.3 Build 20191026 rel.13901(5553)

I don’t see where any of these settings should matter to the 5GHz-1 Wi Fi but figured I would list them.

I bought a three-band router but I can only use two of them.