Can't enable SQL Server 2016 Database Mirroring

I'm having what appears to be a pretty standard issue - only the default answers don't seem to be resolving it.

I'm trying to setup DB Mirroring for the first time - I have 2 servers, one on-prem, one in Azure without a Witness. They are connected over VPN both on Domain (according to Firewall,) and both can see the other one using fileshare.

I'm getting error :
Database '' cannot be opened. It is in the middle of a restore. (Microsoft SQL Server, Error: 927)

- I've done a full backup from the primary and restored it to the mirror with NORECOVERY
- I've done a log backup from the primary and restored it to the mirror with NORECOVERY

- Wash and repeat (a lot of the solutions revolved around just a flaky backup so I've tried multiple times.)

The DB on the mirror shows as (Restoring...). Both servers are running as the same service account...