Hi, guys. I tried adjusting the ports as above and it made things a little better but still experiencing significant lag.
I'm thinking it may not be connectivity related? I've noticed that most days in the game kick off with a long stretch of heavy lag in a split screen game, during which time the wheel and pulley icon appears in the bottom right corner. The game labours until the pulley disappears (which in some seasons can be half or most of the daylight hours so pretty crippling).
I don't know what the game is doing at this time but it shouldn't be grinding the game to a halt.