At the moment trying to enable Self Sign Up with no domain restriction in OnDemand is not possible.

Selecting 'No Restriction' and clicking Save will show the Loading spinner, then fails to save with no warning.

using the Network inspector shows the following request gets made to UUM:
base_URL/admin/rest/um/1/signupoptions

However the server returns with a 500 error:

type Status report

message Internal Server Error

description The server encountered an internal error (Internal Server Error) that prevented it from fulfilling this request.

Workaround

Please contact Atlassian Support to have a workaround applied to your instance.