@The-Burro_1. 

Hi, Are you intended to replace the old Arris (Hiltron) Modem with a new Arris Surfboard, model SBG10?

If yes, you could have a try of the following network layout:

Spectrum 400m/s cable service---Arris Surfboard(working as a wireless cable modem router, no need to put it into bridge mode)---Linksys 5 ports switch---main W3600(in AP mode))))(((( 2nd W3600

Spectrum 400m/s cable service---Arris Surfboard(working as a wireless cable modem router, no need to put it into bridge mode)---Linksys 5 ports switch ---Archer A4400(also in AP mode)                                                                                                                                                                                                  

And if you have Ethernet ports around the house, you could also wire the second W3600 into the switch to create an Ethernet backhaul between Decos.

Since both Archer A4400 and Deco W3600 could work as wireless routers and access points, they are basically replaceable and when working as access points, it could avoid the double NAT issue.

  @David-TP The Archer won't reach the Surfboard remember it's on the second floor unless you were saying that it could effectively cover the entire upstairs, which would leave me with Surfboard, Main Deco, 2nd Deco (possibly downstairs?), and the A4400 upstairs?

If you get some time perhaps in a bit more detail, please. Much appreciated.

Burro

  @The-Burro_1. 

Thank you for the update.

So basically, there would be no way to get an Ethernet connection upstairs right and the AP on the second floor have to be wirelessly connected to the AP on the first floor?

there is a rough picture of the network topology:

As for how to set up Archer AX4400 as an access point and please have a look at this link:

How to configure the TP-Link wireless router as Access Point?

  @David-TP Correct no way for Ethernet or Coaxial connection upstairs I initially thought the 4400 would have been connected to the Surfboard or the older Arris and the Decos would have simply extended that network to the rest of the home. 

  @The-Burro_1. 

Thank you for the update.

Since Deco would not wirelessly connect to AX4400 or Arris Surfboard as an extender, the main Deco has to be put downstairs and wires to the switch or modem.

Currently, we could refer to the above topology and test the performance of AX4400 as an access point to see whether it could improve the overall performance 

  @David-TP I can also omit the Surfboard and simply stay with the Arris modem I have in use, but the A4400 has enough LAN connections as the switch so it's a shame that I can't have that bad boy coming directly off the modem due to its power and capabilities. 

If I had simply just bought the A4400, is there any TP-Link mesh wi-fi 6 hubs/products that would work with the Archer or would I simply be better off just adding to the Decos?

All of this no compatibility has me extremely frustrated. 

Thanks for all your input, 

  @The-Burro_1. 

Hi, as for the mesh compatibility, you could have a look at this link:

TP-Link OneMesh™ VS Deco Mesh: What's the Difference?

So they cannot build a mesh network together.

 For using Archer AX4400 as the OneMesh router, there are some compatible OneMesh extenders, such as 

https://www.tp-link.com/us/onemesh/product-list/#range-extenders

 And the mentioned network layout:

Old  Arris (Hiltron) Modem----Archer AX4400---<wired>-Deco W3600(downstairs in AP mode))))(((( Deco W3600(upstairs in AP mode)

It also works if you want to set up Deco W3600 as a wireless router and just the recommended AP mode could avoid double NAT issues.

And comparing with OneMesh combination Archer AX4400 with OneMesh Extenders(take RE600x for example and you might also need two RE600X), I would vote for a three-pack Deco W3600.

Thank you again.

  @David-TP Another quick question. I have the Main Deco going into the Router and also into the Linksys switch so that everything that is hardwired into the switch is connected to the modem.

We've been having a ton of daily offline red Main Deco alerts as to where I have to optimize the network to get everything to work again which is in no way normal. Does it matter what port on the main deco the router and the switch are connected to?

Thx, Burro