the only way to assign a managed identity is via the PS or az cli, command below, the UI role assignments for example “Owner” are only for the managment. For any roles with the data plane should use the following.
readOnlyRoleDefinitionId = ”
principalId = ”
az cosmosdb sql role assignment create –account-name $accountName –resource-group $resourceGroupName –scope “/” –principal-id $principalId –role-definition-id $readOnlyRoleDefinitionId