@Sunshine not sure what you mean by topology. But I had a fully working system. Then the router died. So I got the Deco X50 system. Nothing on the 2.4 band could connect to the internet. Could not connect at all, or connected with no internet. The 5 gig band worked fine. Eventually gave up on the Deco system and put on a cheap TP-link router I had lying around. Put that on the 2.4 band and then connected the Deco X50s in access point mode to that router. I put the Deco on the 5 gig band and now everything is working. All of my IoT stuff is on the router, the phones and tablets on the Deco. The problem I seem to be having is the IoT stuff is not fully working. I have around 30 or so Wifi based devices. I cannot add any new device (simply will not pair). I have tried re-pairing existing devices that are not connecting. No go. So I turned on the IoT channel on the Deco and these previously working devices now seem to connect with the Deco. It looks like my cheap Tp-link A6 router cannot handle more than around 25 devices. I understand that this is a fairly common problem. The Deco cannot either, so am thinking to get another router that will. Hence my question here. 

I have a lot of stuff on Zigbee and I will be transitioning over to that eventually. I really only need a router that can handle 40 devices tops. There's nothing else on the 2.4 band except IoT stuff so it seems an easy ask, but very few routers even mention this limit. 

I have a couple of repeaters on the 2.4 band (EasyMesh) and have 100% signal everywhere. And 5 Deco units on 5 gigs. Only 1 hard wired back channel on the Decos. Not ideal, I know. 

I am thinking a better router might fix my problem. I hate to dump the Decos as they cost me $700 but it doesn't look like they are designed for IoT at all. 

  @Sunshine   Thanks for the response. The 32 device limit on the Archer A6 router is roughly the limitation that I had worked out as well. Good to have it confirmed. I think the Deco X50 has a similar limitation.

And of course I tried to connect all of the IoT devices to the Deco system. They are all sitting out there online 24/7. I cannot go around and turn them on or off (at least, not easily). And that is where everything fell over. I had everything working fine on one system (the Telstra one). That died, so went out and bought the Deco X50 and plugged it in with the old SSID. Did not work and seemed to 'kill' the 5 ghz system as well. Totally overloaded the Decos, I am guessing. Took me a few days to work out what was going on.

Bit puzzling as the Decos are pretty expensive ($700) and they even have a dedicated IoT Wifi channel. You would think that they could handle a fairly small IoT system like I have. 40 or 50 devices is a pretty small system (and I don't even have that on wifi). You can easily get into the hundreds of devices.

I now have a TP-Link AX20 router plugged in to replace the Archer A6. Still not a great router, but I think it has a better capacity for devices. Time will tell as I have only had it online for an hour or so.

That is the reason for this post in the first place. The specifications listed rarely shows how many devices a particular model can handle. It would be wonderful if as part of the specifications they listed these limits. IoT networks are becoming very common after all. Hopefully a fellow user has found that a certain router works well and would share that knowledge. If anyone asks me, I will certainly pass on the knowledge that the Deco X50 system is no good for this, nor is the Archer A6. 

Do you happen to know how many devices the TP-Link AX20 can handle? I still have a few devices not connecting, so am not sure what is going on. I am happy to buy (yet another) router - I just need to know if it can handle (say) 50 devices. I am looking at the AX5400 in particular. Do you think that would work with that many devices?