Previously you mentioned that you performed a Factory Reset. That should have also done the Re-registration on the Indoor sensor. The fact that the initial Factory Reset didn't correct the problem indicates to me that you may not be running the latest firmware. And do keep in mind that there are two different firmware that need to be performed, each with their own method. Update the WiFi Firmware with the WS View mobile app and then don't forget to also do the download firmware upgrade with the microSD card. You can download the firmware file from Ecowitt.com ...the one for the HP2551 is what you need (yes it works...same system). So be sure to do both the WiFi Firmware upgrade (WS View app) and the Revision Firmware upgrade (microSD card file download).
For the initial problem I would never have suggested a Factory Reset. But you mentioned that you had already done so. What is done is done.
There is a clear explanation for your initial problem. If you removed or replaced batteries in the indoor sensor then this sensor will randomly assign itself a new ID number every time the batteries are removed and reinserted. The display console needs to re-register and search for the newly assigned ID.