@my2108 

You can't combine them in one mesh. They can run together and complement each other: turn off WiFi on your AX1500 Router and run Deco M9 Plus in Access Point mode. 

In that configuration, you can seamlessly use all four Ethernet ports of AX1500, its network management features. Deco will provide WiFi coverage. 

  @Alexandre. Tks for your reply.

In case i need to add additional node into the mesh network, I assume I can add any variant of Deco series..am I right?

Also, in case in need to use a powerline kit like TL-WPA8631P KIT to punch thru walls, shall I be connecting it to my Deco M9 or the AX1500?

  @my2108 

In case i need to add additional node into the mesh network, I assume I can add any variant of Deco series..am I right?

 

Any variant of Deco series can run with M9 Plus, but if you want Deco mesh to run well these are my recommendations:

1. If you plan to keep your Deco M9 Plus, which is WiFi5, add to them Deco hardware that is also WiFi5. Don't add WiFi6/WiFi6E/WiFi7 capable hardware. Stay on the same WiFi protocol. 

2. From WiFi5 Deco series only M9 Plus is tri-band. If your additional Satellite Deco(s) will be connected wirelessly to the rest of Deco mesh, go with more M9 Plus. The WiFi speed difference between tri-band and dual-band Satellite Deco is noticeable.

3. For Ethernet wired Satellite Deco you may consider Deco M5. Deco M5 will work well with your Deco mesh, as long as it is wired and you need it to provide coverage for smaller spaces than M9 Plus units. You can also buy single M5, but M9 Plus are only sold in pairs and sets of three. 

4. Both Deco M5 and M9 Plus are the only WiFi5 series that come with firmware 1.6. That new firmware has some very nice features described here: Deco M5 1.6.1

Some of these feature only work if all Deco nodes in Deco mesh are on firmware 1.6, add older firmware version Deco and they'll be disabled. There is no information when will other Deco WiFi5 series get firmware 1.6. Makes sense to stay with M5 and M9 Plus.

In summary, your best choice for additional Deco nodes is between M9Plus and M5. If you will need more than one additional Deco unit, I would suggest to add more M9 Plus. If you'll only need one more and it'll be Ethernet wired, M5 is an option. If you have multiple small rooms to which you'll bring Ethernet or powerline, M5s are also options for those rooms.

My Deco mesh is two Deco M9 Plus and two Deco M5. Main Deco M9 Plus, Satellite Deco M9 Plus wirelessly connected to Main Deco, two Satellite M5s both Ethernet wired. Deco mesh runs in AP mode.

Also, in case in need to use a powerline kit like TL-WPA8631P KIT to punch thru walls, shall I be connecting it to my Deco M9 or the AX1500?

 

With Deco mesh in AP mode it would not matter. You can connect TL-WPA8631P to either. For powerline adapters to work well, they must be on electric outlets that are on the same circuit breaker. Find electric outlets that are on same circuit breaker and connect TL-WPA8631P to what it is closer to: Deco unit or AX1500. 

TL-WPA8631P provides own WiFi signal and it will not join Deco mesh. It'll run its own WiFi network, even with same SSID.

You also have different option: buy a pair of regular TP-Link powerline adapters and attach Deco M5 to one of them. That may result in better WiFi roaming experience.

  @Alexandre.  WOW, your answer is superb, thanks very much for your help.

You seems to suggest Deco M5 needs to be Ethernet wired, but afaik M5 is also capable of form wireless mesh without wired Ethernet connection?  Am I missing anything?

You also mentioned TL-WPA8631P KIT form its own wifi without joining the mesh network, but i thought this device is supporting the latest OneMesh Wifi?

I once tried setting up same SSID with wifi extender of another brand, and it worked horribly.  So I have a very bad impression on non-mesh wifi extension.