Does the 1080p resolution show up if you go to Advanced > Show Resolutions as List and then tick "Show all resolutions" under the list? The resolution you are looking for is probably 1920 x 1080 (low resolution). If you choose a non "low" resolution the OS will output at 4K but scale the UI to the virtual resolution.
I still won't forgive Apple from dropping 1080i resolution from macOS. I've got a home theater Mac mini hooked up to my TV which only supports 720p or 1080i. I've always displayed 1080i fine, and then one upgrade later, it's suddenly "Fuck you, user. Use 720p LOL."
