First of all, please check that apps are indeed available in your LAN.
To test this, find Headwind MDM launcher in the "Applications" settings and get its URL:
Replace the domain name in this URL by the local IP address and test if it could be downloaded.
If you are able to download apps through the local IP address, replace the URLs of required applications.
For each application which has the URL, click the "Versions" action.
Then click "Edit" and update the URL by changing the domain name by the local IP address.
The devices should be now set up through the local IP.
After you setup all devices, revert the URLs back to the original domain name.