Thunderbolt Display disabled after upgrade to Tahoe 26.5

Users report the Apple Thunderbolt-3 <-> ThunderBolt-2 adapter performs admirably, and no reports of failures of that device have been seen here.

There have been reports after various upgrades that the stored preferences from an older version of MacOS interfere with a current version. This is not supposed to happen, but there is a work-around:

For MacOS 12 Monterey and some later versions, some users got relief by deleting some preferences files, then Restarting.

To access the folder using Finder for the system-wide preference

Copy and paste this string into:

Finder > Go menu > Go to Folder

/Library/Preferences

then find and delete this file:

/Library/Preferences/com.apple.windowserver

-AND- To access the folder for your-user preference

Copy and paste this string into:

Finder > Go menu > Go to Folder

~/Library/Preferences

then find and delete whichever of these two files you find:

~/Library/Preferences/com.apple.windowserver.<UUID>.plist -OR-

~/Library/Preferences/ByHost/com.apple.windowserver.<UUID>.plist

-- where [~] tilde is an active abbreviation for "current user"

-- and <UUID> is a string of hexadecimal numbers unique to your system, grouped as 8-4-4-4-8 hexadecimal digits

Restart REQUIRED after these deletions.

recent experience by other users suggest the files you might find at:

~/Library/Preferences

could be any of:

then find and delete whichever of these files you find:

~/Library/Preferences/com.apple.windowserver.<UUID>.plist   -OR-

~/Library/Preferences/ByHost/com.apple.windowserver.<UUID>.plist   -OR-

~/Library/Preferences/ByHost/com.apple.windowserver.displays<UUID>.plist 

Thunderbolt Display A1407 sold 2011 through 2016, with Thunderbolt-2 and MagSafe (and no others) in its captive connecting cable?

How are you connecting that to a Mac Mini?

Are you using an Apple keyboard or another brand? Other brands may require to use the control key when using the brightness key on the keyboard