Then, looking at the page linked above, you likely have a problem with the yellow/blue wire at the TGPI switch on the rear of the transmission. The problem could also be farther up the circuit, e.g. in the gauge cluster, the wiring harness, or the connector to the cluster.
I'd pull the connector for the TGPI switch, which you may be able to do without removing much if anything from the bike, and test for continuity as indicated on that page. What you're likely to find is no continuity between the yellow/blue and brown wires in any gear. Per the chart there, you should have continuity in neutral, second, and fourth.
If the wires at the switch connector behave as they should, connect them back up and do the same test at the cluster connector. Pins 2,3,4, same colors. If they work at the cluster connector, the problem is inside the cluster (or with the pin-to-socket contact at the connector itself).
Edit: One other thought. IIRC, that connector is the same kind of four-wire plug as the fuel tank connector, which is located very nearby. (The fuel tank connector should be clipped to the front-right of the under-seat tray.) If so, the connector itself is notoriously prone to failure, as the pins move around in the squishy rubber that's supposed to hold everything in place. So, if you test for continuity at the connector and it seems fine, try carefully and firmly re-connecting the connector, and then just check your display running through the gears. Re-seating the connector might resolve it.