Why can't I see OGC layers displayed in ArcGIS Online?

There are two map viewers currently available: ArcGIS Map Viewer Classic and ArcGIS Map Viewer. You can try with both; we noticed that layers are more likely to work in the new ArcGIS Map Viewer. You can also try to use WMTS instead of WMS, as it’s expected to work better.

ArcGIS also suggests to configure its CORS settings to allow specific domains to communicate with the server, as described in this ArcGIS Online documentation.