If using Thingworx Integration Runtime version 8.0.9 or earlier IR versions with Windchill Rest Services 2.1 then errors can occur.
PTC strongly recommends to upgrade to Thingworx Integration Runtime version 8.0.10 to work with Windchill Rest Services 2.1. But check ThingWorx Navigate Product Compatibility Matrix and confirm if IR 8.0.10 version is supported and compactible with current Thingworx Navigate version before you proceed for upgrade . If current Thingworx Navigate version is not supported with IR 8.0.10 version then upgrade Thingworx Navigate to latest Navigate 9.1 version which supports IR 8.0.10 version.
However, my TW IR is 8.5.3 and Rest is 2.1 so there should be no issue.
So I am a bit confused on what to do to make this endpoint work.
From the OData Utilites I can easily use the v5/ProdMgmt service endpoint just not through Thingworx Service Definition.
You mentioned that you're running ThingWorx IR 8.5.3, but that sounds like the ThingWorx Platform version. Can you please confirm the ThingWorx version and provide the Navigate version you are running?