You can use the following tool to discover Grandstream IP address on your network:

Once you have the IP addresses of both the GWN7600_LRs, make sure they are on the latest firmware.

To do this go to "System Settings" > "Maintenance" > "Upgrade" and paste the following link into the "Firmware Server" section.

Once you have done this click save and apply and then the "Upgrade" button.

After around 5 minutes the unit should have finished the upgrade.

Once you have completed the firmware upgrade. In the same section there is a "Factory Reset" button.

In the GWN7600_LR that is going to be at the cabins end, click the "Factory Reset" and wait another 5 minutes.

Now go to the GWN7600_LR that is at the main house.

Under "Access Points" select "Discover AP", you should see the second AP show in here.

Click the links symbol and you should see the second AP appear as another device.

Make sure both the APs are members of the SSID group.

Go to "SSID" and click on the edit button. Under "Device Membership" make sure both the devices are "Member Devices".

Click "Save".

Finally go to "System Settings" > "Mesh" and tick both "Mesh Enabled on 2.4G Radio Interface" and "Mesh Enabled on 5G Radio Interface".

Click "Save" and "Apply" all of the changes.