Unifi UDM wifi issues

I bought a Unifi Dream Machine (not UDM pro) which is a sort of all-in-one unifi system.
In general, I didn’t have that much issue with my previous network devices, but wanted to have more control over my network. Couldn’t afford to buy UDM pro + poe switches + APs etc.

I watched a few videos and read some documents and set it up.
What I did is based on The Hook Up youtube channel, this video . This means I created an IoT for devices like google assistants, and NoT for all other devices that are supposed to be local such as diy ESP-based devices, some tuya plugs (running localTuya), etc. all on 2.4GHz only.

I have no wifi interference with neighbours and already chose the channels that do not overlap with my Zigbee networks (the same as my old setup).
However, the wifi devices on the NoT network get unavailable almost 50% of the time. 10 minutes they are working, 10 minutes some of them randomly go unavailable - regardless of their distance to the UDM, some of them are just a few meters away from UDM.
I should mention that I used to have this problem of going unavailable before with my previous wifi router. But they were like 1-2 seconds which I could never see unless I looked at the device history. But now with this new UDM they go unavailable for long periods. Honestly, the reason that I bought this UDM was to get rid of those occasional unavailabilities - well now it’s super worse.

So I was wondering if anyone has any solutions or points that could help me identify the problem.

By the way, the network architecture is:
The ISP modem is connected to the WAN port of UDM. UDM has 3 wifi SSIDs (main, IoT, NoT). One ethernet port of UDM is connected to another unmanaged switch and that one is connected to likes of home-assistant server (rpi4), a NAS, another unmanaged POE switch for cameras, Blue Iris NVR, etc.
Any help would be appreciated.

[The reason that I’m writing it here in HA forum rather than Unifi one, is I thought it might be a mixture of HA and Unifi issue - not sure honestly]

It is important to know if device actually disconnected from network.
From description, I assume devices on wifi are having issues. Do you see them reconnecting to wifi? Can you do a constant ping test to device(s) that fail? Make sure you don’t have any address conflict.
In addition,are your networks 2,4 or 5G? Can uou dedicate IoT to 2G only?

So many thanks for replying.
Yeah you are right I should take a look at the devices.
For example, at the moment one ESP device is disconnected and I can ping it, and can go to its web page and it’s working by its own. It reports wifi signal of -64dB. UDM reports its experience from 75% to 99% (it changes). Sometimes even it disappears from UDM device list and quickly comes back with 100% experience. I did a few minutes of ping, it pings from 1ms delay to less than 1000ms - once or twice time-out.

IoT network ( the one that has google home devices) has internet access and is 2.4/5G.
NoT network (the one that is connected to all local devices such as ESP based devices) is 2.4GHz only. Is it what you wanted me to do?
I will take a look at the unavilable devices more and report back.

I just saw that all devices on 2.4GHz network became unavailable - both on IoT and NoT.
Then I pinged a few of them and they all were unreachable.
Afterwards I opened the WiFiman (unifi’s wifi analyzer app) on my phone, and noticed all SSIDs om 2.4GHz have disappeared (but not the 5GHz ones) and ESP devices turned on their fallback hotspot SSIDs.
I kindda feel there’s something wrong with UDM’s 2.4GHz radio. Any idea?

But you did have similar issues before, that’s why you switched to UDM right? And you keep having those issues even now?
Connect PC to that wifi and ping constantly gateway. Does it get disconnected too?
Because at this point, it seems like your issues are unrelated to HA, just to network. And you need to figure out whether it is few devices that think that lost wifi, or is it actually that SSID that disappears, which could indicate UDM issue, but still, if that was the case, any device connected to it should fail.

Yeah, apparently it’s a udm issue. At those occassions all 2.4 SSIDs disappear but 5g stays on.
With my previous device I had a few unavailability for all iot devices, but each unavailability lasted a few seconds and I didn’t notice it excpet if I looked at the logs. It was an old router and expected probably.
But with udm it’s some times one or two hours devices go unavailable now.
The very least thing happening here is udm’s 2.4ghz radio goes offline for whatever reason.