dotnet restore fails in docker on 1 wifi, ok others

I have a local build of my dotnet core app that runs in the microsoft/dotnet:2.1-sdk image. The dotnet restore part of this works perfectly fine on most connections but there is 1 wifi connection where I work and I get errors like:

Retrying 'FindPackagesByIdAsync' for source ''.
The SSL connection could not be established, see inner exception.
Authentication failed because the remote party has closed the transport stream. 

If I try using curl to get the json file by manually running the image, it works ok:

PS C:\Dev\src\Deployment> docker run -it 6baac bash
root@99660525f031:/# curl 
flatcontainer/system.buffers/index.json --ssl
  "versions": [

I tried to 'forget' the connection and re-establish it. Ive tried re-installing Docker. Ive tried standing on my head. Nothing seems to work - unless I move to another location and use an different wifi connection (or my docking station which I left in another country).

Does this make any sense to anyone??