@JasperCat 

In regard to Deco X20 lights:

Deco X20 devices have LED indicator lights that cast a glow from underneath. When it’s solid yellow, the system is starting up, followed by a pulsing blue light to show it is ready to be set up. When the light is green, everything is OK, but if it blinks red, the device has lost its Internet connection. 

Which means, your Main Deco and Satellite Deco should have green LED lights after successful setup.

I can give you the steps on how to set up Deco mesh with ease (or, at least, it'll be easier than how you did it) and with high probability of success on first try, but for that, I would need to know the current state of your Deco mesh:

1. Do you see all Deco nodes in Deco app or have you deleted them from Deco app?

2. If Deco nodes are in Deco app, do they all report as online?

When you turn on bridge mode on your ISP router what it does is (at least on all ISPs routers that allow this) is to define a specific port (usually port 4) as WAN (internet) port (direct internet to this port, a bypass outside of the router network), giving you 2 external (internet) IPs: one to your ISP router, another that you can use (with your Deco). Usually a restart of the ISP router is needed for this to happen.
When you connect your main Deco to this particular bridged port, your deco bypasses the ISP network and gets a WAN port.

Nothing else changes on your ISP router, so your ISP router network and wifi will stay available if the later is not specifically turned off.

So you need to be sure to plug your main Deco to this particular bridged port, any other port will connect your Deco to the ISP network.

And if you want to setup your Deco network using the same network name (SSID) and same user/password, you need to turn off your ISP wifi before setting up the Deco network, otherwise they will clash and your clients (and Deco satellites when setting them up) might connect to the wrong ISP network instead and cause havoc, use your mobile connection instead for the setup.

And remmember that if, at any time, somebody turns the ISP wifi on, you will get havoc again. You need to know well what your are doing, so my suggestion is to create a diferent network name (SSID) on the Deco.

@Alexandre. 

I have deleted the Deco App.

@JasperCat 

If you haven't returned Deco units and want to try again, you will need to install Deco app. You can do that on same or different smartphone, but you must login to the app with same email address/password you used for the first try. Your Deco nodes do belong to Deco mesh you've created, and that Deco mesh is managed by Deco app under that email address you used first time.

Then, you can answer my two questions about current state of your Deco mesh, and afterwards I can provide next steps.

@Alexandre. The app is wiped clean of my previous set-up.  It doesn't see anything and prompts to start the set-up the again.

@VCL 

My Xfinity combo modem/router has 4 Ethernet ports. How am I supposed to know which one will work? They are labeled 1 and 2 --one row for "phone symbol" and one row for "computers symbol".

I saw a video on YouTube by a guy saying that Comcast never told him that there is in fact, only one port, he could use when placing the combo modem/router into bridge mode. I'm assuming the lower left one...?? Which is similar to what I saw referenced in the YouTube. video.

If I knew how to do a trace route command and had a computer with an Ethernet connect, would that be the only way I could tell if it's a true bridge?

JasperCat wrote

@Alexandre. The app is wiped clean of my previous set-up.  It doesn't see anything and prompts to start the set-up the again.

@JasperCat 

Try set up again. This time, when asked, select different SSID than you have on your home WiFi network. It is important. Later, there will be step to transfer Deco mesh to your original SSID, but now you should have different SSID on Deco WiFi so that your home WiFi you now have from Xfinity stays undisturbed.

For setup:

• Do nothing with your Xfinity router
• Connect Deco unit to Xfinity router Ethernet LAN port by Ethernet cable
• Power it on, proceed through setup and add that Deco unit to Deco mesh as Main Deco
• If that works - continue with adding two remaining Deco units, these do not have to be connected by Ethernet cable, but have them near Main Deco during setup

If you have completed that successfully, you should see three Deco units in your Deco app, all report online and every Deco unit has green LED. Power them off. That will be all for now.

@Alexandre. 

Oh wow. So DON'T but the Xfinity modem/router into bridge mode? Won't the two routers fight?

JasperCat wrote

@Alexandre. 

 

Oh wow. So DON'T but the Xfinity modem/router into bridge mode? Won't the two routers fight?

@JasperCat 

Do not put Xfinity into bridge mode.

I am guiding you through the path where you do not destroy your current WiFi network before Deco mesh replacement is ready to take over. Which means, at any step if something is not working with Deco you still have your WiFi running.

Also, if you decided to give up on Deco for some reason, during that process, you wouldn't need to call your ISP tech support to get your Internet back.

Yes, two WiFi routers in close proximity from each other is not good, but for a short while it is not very bad either. One of the reasons I told you to power off all three Deco after you have set them up with different SSID - is to reduce the time when Deco WiFi and Xfinity WiFi create interference with each other.

Also, having different SSID for Deco will ensure no WiFi device will connect to Deco mesh until that mesh setup is complete.