The Helix Fi app does not have such feature but you can use the Helix gateway's admin page at http://10.0.0.1 to assign a static IP on your devices.
To log in, the default username is set to 'admin' while the default password is set to 'password'. If this does not work, there may be another password set and in this case, the only way to solve this is to do a factory reset on the unit.
When you have access to this page, you must select "Connected devices" in the menu on the left. Then use the "Edit" button next to the device of your choice in the section on the right. Lastly, check the "Reserved IP" box and specify the IP address of your choice using the same format as the gateway IP address ( 10.0.0.X ) then save your settings.