@R3adyPlay3rOn3 

Deco does not extend WiFi network of your router, even if you run Deco in Access Point mode. What you have done is created two different independent WiFi networks each broadcasting the same SSID. This is what might make google devices confused.

The first step would be to turn off WiFi on your modem/router/gateway combo, to prove that theory. If you can't turn router's WiFi off, change router SSIDs to something different so that no device in your household can connect to them.

If this does not help, it may be that your google devices do not understand mesh or band steering. It would help if you can tell what these devices are, perhaps it is known issue for that type of devices and workarounds already been discussed in this forum or on google support forums.

@Alexandre. 

Thanks a lot for replying! That is a good idea and I will temporarily change the SSID of my ISP providers gateway combo and see which devices connect and work properly with the Deco mesh system alone and then report back. 

My confusion came from the fact that when the Deco is in AP mode it doesn't do any DHCP server functions like assigning devices IP addresses,  and since that is bieng handled by my ISP provider gateway I thought my end point devices would just see a wifi SSID and connect to it and the IP assignment would be handled by my ISP provider gateway... ultimately causing no conflicts.

But let's test your theory first which is a good idea and I'll report back shortly.

My Google devices are a Google chromecast audio, and an Insignia google home speaker. I'll double check and search the forums for them for known issues.. thanks again!

Update: I changed my ISP provider gateway SSID and the good news was that all devices worked but the bad news is that my Google home mini and my Insignia google speakers both ended up dropping the connection eventually. They do work just fine after a reboot though. So what I've done as a final test is I've disabled the "mesh" setting on one Insignia google home speaker and left it on, on the other one. If one stays connected then I know it's just a toggle of the mesh setting for these 3 devices. Or else it's getting returned.

@R3adyPlay3rOn3 

There is another option you could try: connect these google devices to new SSID of ISP provider gateway. These are stationary devices, I am guessing, they don't need to travel seamlessly between WiFi nodes.

With Deco in AP mode, it does not matter where they will be connected, as they will be part of the same network with any other device connected to Deco.

It appears these google devices do not understand WiFi mesh. Replacing Deco WiFi mesh with some other brand of WiFi mesh won't fix that, but why not to try and see for yourself, if nothing else works. 

@Alexandre. 

Toggling the "mesh" setting to off did not fix the issues and my insignia ns-cspgasp still eventually loses connectivity. 

I rely on this device for home automation so it not working with the Deco is unacceptable for my needs and I will return it and give the Eero pro a try. 

I could always do as suggested and rename my ISP SSID and connect it to that and see if it works but I shouldn't have to and should be able to use one SSID in a mesh setup. The last I could try is turn on router mode and see if it stays connected but even if it does then I'd be back to my original problem of not being able to set my DHCP server IP range..

@TP-Link 

Thanks for replying. Just so that I am clear, are you suggesting that I temporarily disable the 2.4ghz band on my deco x3600 (x86) and see if that help solve the connectivity issue of the Google chromecast audio & the insignia ns-cspgasp? 

Because if you are not saying that but suggesting that it could be my ISP gateway causing 2.4ghz interference I'm not sure how? That is broadcasting a separate SSID altogether and it's like saying my neighbors signal could cause my connectivity problems...I don't think that's it.

I really want to like this device because outside of the 3 devices not staying connected it works fine.

@TP-Link 

Okay thanks I'll give that a try next. I did try one other test (which I am not sure is the solution but worth a try) and that is I changed the wifi encryption methods settings. I am wondering if this device doesn't support a certain type of encryption but what makes me believe that this isn't the problem is that it most likely wouldn't connect at all in the first place if it didn't support that... but I made the change to test and will report back.