How to cascade (bridge mode) connect a Linksys Smartwifi router as a secondary router

In this post, I will explain and give a real world application of how to piggyback a Linksys Smart Wifi router with the new 2019 firmwares you have to deal with.
Why?  There are a lot of changes with the new smart router firmware, and no where on the internet, including the manual or Linksys' Website, explain it well.
For this documentation, I use a WRT1900AC and an EA6400.

First, I want you to read this.  This was the closest thing I could get to helpful:
https://www.linksys.com/us/support-article?articleNum=137888
If you are trying to expand your network with a Lan-Lan connection, forget all you ever knew about the traditional Lan-lan setup.
Linksys has made it easier for the newbie and harder for the experienced networking administrator!

https://www.linksys.com/us/support-article?articleNum=137888

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Things didn't go exactly like this: you have to navigate to www.linksyssmartwifi.com. Not 192.168.1.1 or myrouter.local anymore!
And you have to wait a minute for the router page to come up all the way.

1. Reset router. If its not reset, you won't be able to proceed.  For instance, on the WRT1900 AC, there will be no option to change the internet type.  YOU MUST RESET the router.  No one else on the internet will tell you this important step (took me 6 hours to figure it out)
2. Hook up router ports 1-4 on the secondary router to a computer's ethernet.
3. go to www.linksyssmartwifi.com on the pc
4. Login to the secondary using the default which is admin, admin
5. Go to connectivity, internet settings, IPV4, edit, Bridge mode
6. Specify the IP address so that you can pull up the router later easier. For me, the main gateway has an IP address of 192.128.0.1, so I had to set the secondar to IP address: 192.168.0.2, subnet mask 255.255.255.0, default gateway: same as main router.  
7. Click OK, and save the settings.  YOu will now loose connectivity with the router.  
8. Connect the modem-router (gateway) to the Internet Port of the Linksys router.
9. Once your router detects that there is Internet access, it will give you an Internet Restored prompt.  Click Yes. IF this doesn't happen, try resetting everything. For instance, if the gateway modem is not giving out any ip addresses, then reboot that. Login again to www.linksyssmartwifi.com

9. Setup your router wireless settings. You won't be able to do much, no more parental controls, etc. Its just a bridge now. If you have a usb device plugged in, it will just show up on the network. You can no longer control its access, etc.  This is your last change before you unplug the computer you are accessing to the router locally with and installing it on the network.  Otherwise you will have to connect directly to the router later (via wifi or ethernet) and made changes like the wifi password.

10. When done, leave the secondary routers Internet port hoooked up to ports 1-4 on the primary. Smart routers like this Linksys do not need you to change it to a port 1-4.

EA6400 was able to change to bridge mode without resetting. However the AC1900 did not have that option until I reset the router.   Was trying to configure like this through another router – doesn't work. Also remember, you Can't backup settings except when  locally connected to the router.