In DHCP, the DHCP server allocates IP address to devices that request them using an algorithm that usually results in the device receiving the same IP each time it is requested. But it is not guaranteed to always be available, so over time, another device might receive it if the original requestor is offline for an extended period.
To ensure a deice always receives the same IP via DHCP, you can create what is known as a ‘Static Lease’ and lock in their IP, and the router will always hand that IP to that device and never to any other device.
To set it:
Log into the IQrouter, then Select Advanced->Advanced
Go to the Network->DCHP and DNS page
Note current IP and MAC address of the device you are targeting in the Active DHCP leases list
Click ‘add’ static lease to create a new blank entry
Fill in a name (e.g. Thermostat) then pick the MAC address of the server and for IPv4, pick the IP of the server
Click ‘Save & Apply’ button.
Note1: you can directly address any device by its DHCP hostname (shown in the list), this makes it more convenient than the IP. So if the device already has a reasonable name, you can use the ‘hostname.lan’ to reach it.
So for example, a thermostat has a hostname displayed as ‘tstatAB34’ you could reach the web UI for is by typing http://tstatAB34.lan in the address bar of the browser. So regardless of the current IP address assignment via DHCP, that will always work.
Note2: If you have a device that does NOT have a self-assigned hostname (displays as a ‘?’), then you can use the static lease entry to give it your own hostname, and then be able to use the name as indicated in Note1.
An even if it has a name, you can rename it using the static lease and set your own custom hostname.