Hi there,
There is no alternative per-se. It’s a network thing.
As you well guessed, you can only take a network perspective approach, forcing the users to log from a single ip or ip range.
Options that come to my mind, in addition to the ones you indicated.
- Client VPN connection: either the one of the organization or a new one towards your Viya subscription
- Set an additional reverse proxy with additional rules and MFA (Application Gateway)
- ensure Viya can be accessed only internally, no public ip addresses: Windows Virtual Desktop, Bastion, etc