Thanks for your query.
We've checked around with other forums and managed to find this discussion.
It appears that you need to set an 'Address reservation' via your TP-Link modem/router.
Check the MAC address of your Chromecast. If there's no information on the sticker on the device, then you can follow these steps to get the MAC address.
First, you need to login to the TP-Link modem interface (192.168.1.1). (default username and password are both admin)
Then on the Basic page, click on 'Network Map', then select 'Wireless clients'.
Look for the Chromecast device, then copy or take note of its MAC address.
Once you have the MAC address, go to the Advanced page, then click on 'LAN settings'
Look for 'Address Reservation', then click on 'Add'.
You may fill up the MAC address manually or click on 'Scan', then look for the Chromecast device. Assign the desired IP address for it, then click on 'Save'.
Let us know how it goes.