1. A concise explanation of the problem you're experiencing.
The parameters of the WMTS has been set along with its url, and its loading on cesium imagery with default WGS 84, however data of EPSG 2180 is not landing approriately on Cesium with WGS 84. So this calls for the projection in picture.
There are two ways I think
1) To make the cesium globe ellipsoid 2180 specific and then load WMTS with 2180, to confirm its proper alignment.
2) Or to project 2180 specific WMTS layer to cesium, with default 4326.
GRS80 Ellipsoid = WGS 84 Ellipsoid, with the minute change in flattening only. So creating GRS80 Ellipsoid will not change anything.
How to handle projection and which way in Cesium for this alignment?
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
4. The Cesium version you're using, your operating system and browser.
I think supporting different projections would be a non-trivial task. I don’t know what the step by step instructions would be, but I would take a look at the forum and GitHub (https://github.com/AnalyticalGraphicsInc/cesium/pulls) to references or resources. Here’s a discussion I found about this, I’m sure there’s more: