Hi @digtlpete,
I believe it’s possible to use Google Photorealistic 3D Tiles directly from Google in UE 4.26, but not from Cesium ion. You’ll also have to manually ensure you’re complying with Google’s terms of use regarding attributions, because those old versions won’t handle it for you automatically.
Our ability to provide free support for very old versions is extremely limited, and the Cesium for Unreal plugin has improved greatly since v1.20.1 (the last that supported UE 4.26). So I’m not sure I can really recommend this path to you.