I am trying to add a WMS to a new Web Map as a basemap through a URL. When I toggle on “Use as basemap” it appears to be added under the “Basemap” section of the Web Map, but it is not visible for me. Is there something else I should be doing? Note this layer can be added under the “Layers” section successfully when the “Use as basemap” isn’t toggled.
Talk with your local Esri support (which I am not). I tested your data quickly and if you choose "grid" as a map layer it works as a base. So most likely this whole world coverage is causing here some issues. The BaseMap works nicely as a normal WebMap layer so it is just the AGOL basemap setting that is causing issues here.
My guess is that as service is available over 6000 projections the first one AGOL picks up is just messy and shows you just gray.
If you force service work in certain projection it works also as a basemap. Procedure shown for example in this thread: https://community.esri.com/t5/arcgis-enterprise-portal-questions/adding-external-wms-with-defined-pr...
I tested your url as a basemap using Web Mercator 3857 and it works nicely as shown in attached screenshot.
So my conclution is that there might be some issue with this data when it is used as a basemap and it is worth to have a chat with local Esri support. However there is a decent workaround and it is forcing the service to work in certain defined projection.
Thank you very much, the workaround from the other post worked! This dataset uses the “VTTS_3995_WGS_1984_Arctic_Polar_Stereographic” projection, which is EPSG:3995. I've attached a screenshot of what I put in the "Add Layer" window that allowed me to use this dataset as a basemap in case it's helpful for others.