Is this an OEM product (Polycom) you are trying to re-image? Or one of your own systems?
If OEM product, you have to go back to the manufacture to get the recovery media. Sounds like you did this, but you need to run Windows Update to get the latest fixes.
If this is your own system where you install Microsoft Teams Room, then you will have to follow what YuliSun-MSFT to get a Windows ISO. One could build a custom Windows 10 IoT Enterprise image. It is not that difficult. You will need to get a license from a distributor.