In case others have the same problem, there’s an interesting symptom to try to replicate :-
Method 1.
Everything off (Core & Remote / Remotes).
Core on.
Remote on.
Remote will search until you toggle (on the Core) the “accept Remotes” on / off, or input the Core IP address within the Remote app.
Quit Remote.
Restart Remote - no change to the above behaviour - Remote doesn’t connect unless “helped”.
Method 2.
Everything off.
Remote on (Remote will obviously search).
Core on.
Remote immediately finds Core without any assistance.
Perhaps the “networking gurus” could comment ?