@breeno 

I assume by 100Mbps you are referring to these devices being only 2.4GHz devices.  If that is correct then you can try one of the two things 

1.  disable Fast Roam.  This feature can sometime affect older 2.4GHz only devices or devces that stay stationary. 

2.  Enable the Guest network on 2.4GHz only and see if the devices connect.  If they do then enable the 5GHz on the Guest network and see if they stay connected.  If so then you could add them to the main network doing the same process.  Disable 5GHz and connect devices to 2.4GHz and then re-enable 5GHz

@Carl 

Neither of the devices are running on wifi. They are both 100mbps ethernet connections (and don't have a wifi option as they are IOT hubs) all my wifi and 1000mbps ethernet devices are working just fine. I've tried 3 different switches (2 of them from TPlink) as well with the same outcome. To iterate. this is a new problem post a firmeware upgrade. I have been using the same set up for years with no issues

@breeno 

Okay I didn't understand you were talking about hardwired devices.  In you last response you mentioned using switches.  Did you attempt to connect via a direct Ethernet connect as well, just to rule it out.  I know you mentioned you tried multiple switches but this will be a question the software team will ask when I present this concern to them.  Also are there any logs from the unit you can share when the two clients are connected to see if there is a reason why they are being rejected IPs?

@Carl 

hi Carl. Yes. I've tried connecting directly to the deco with the same outcomes.

the 3 different switches I've used are

1 - Dlink DGS-1024D

2 - LS1008G (I bought to solve an issue between the dlink and the deco's)

3 - TL-SF1024 (I bought in an attempt to resolve this issue)

I've also swapped the cables 

There are no logs from either of the devices unfortunately. Another frustrating factor is that the issue is intermittent. no IP address assignment for a period then it assigns.. some time later (I'm guessing after lease expiry) they are removed once more for another period. One of the devices allows for manual addresses which I have done for now. (whilst I have a connection to it) the other however only supports DHCP. I'll observe if that resolves for at least one device. Neither device supports wifi as they are IoT devices

thanks