SQL Server - Company wants HA for single instance with 1 userDB - sanity check and questions
If your RTO is ~ 1 minute and your RPO is 1 day of data loss then you could use log shipping. This would not require AD, a Windows Cluster or Availability Groups. 

If you shipped your logs every 15 minutes the recovery time needed to bring the secondary should be well under a minute (Needs testing but I would be surprised if it wasn't from what you have explained above).

The only thing you would need to deal with would be how the application manages the connection to the secondary when the primary is down. This could be handled in the application, a load balancer (with static routes that would need to be altered on failure of the primary) or a clustered name resource (would probably need to be manually failed over).

A FCI would mean you wouldn't need to handle and monitor all the log backup/copy/restores of log shipping and your RTO would be lower. This is not possible without AD. Also you can't have Availability Groups without a Windows cluster which also requires AD.