Hi  @jojent 

I don't think it's a switch issue since it's not related to IP assigning.

Do these devices obtain duplicate IP addresses? Can you confirm whether this is indeed the case?

Have you checked if any existing devices in the network are already using these IPs? If so, could you specify what types of devices they are?

here is list of curently conected clients and it only 12 devices so there should be no issues with IP allocation

>>I don't see any duplicate IP addresses. Or did I miss something?

As I mentioned, when the Omada router is removed, the issue persists, this can indicate that the issue is caused by the ISP router. 

Plus, there are two DHCP servers in the network when Omada gateway is added. 

Here is my suggestion: contact the ISP router support to get into the management interface, and turn off its DHCP server, only leave one DHCP server(Omada gateway) for the network.

Or contact them to resolve the DHCP issue under this topology:

Thanks for your understanding.