To get Sergio — the Los Bandidos leader who is best for stealth actions — in Far Cry 6, you have to complete the Tricks Of The Trade mission. And to complete Tricks Of The Trade, you have to complete Undercover Stud, then talk to Gilberto again. If you’re having trouble finding Gilberto, it’s not a glitch. It’s just that the map marker is a little confusing, and if you follow it above ground, you won’t find him. Remember, he’s inside the El Agujero guerrilla hideout, which is accessible via the secret door in the wall mural in Concepción.
After talking to Gilberto, follow the map marker to the Fernando Local Radio Station. If it’s nighttime, remember that Concepción has a curfew, so you’ll need to get out of the town quietly, violently, or quickly. Whichever comes naturally. The posters are on a desk in the same room as Captain Diaz, in the west corner of the building.
Next, you have to find some Admiral Benitex billboards to vandalize. You can either follow the Libertad trail southwest until you reach the road, then follow it around the coast. Or you can fast travel to a location in the right region, such as Hideout Bolero, or one of the Vacía Coast checkpoints. If you haven’t already captured the checkpoints on this stretch of road, you might as well get them during the course of this mission.
Related: How to get Big Papi in Far Cry 6
The billboards are mostly high up, and you may need to get creative in order to reach them. For example, one requires you to climb onto a digger and jump from the end of its arm; another is only reachable using a zipline that starts at a small wooden tower (and you have to detach at the right time). The final billboard requires you to jump from the top of a shipping container.
Once you’ve vandalized three billboards, Gilberto will call, and you’ll get the map marker for the prisoner swap. Follow the marker to Basilica de la Vergen and tag as many soldiers as you can with your phone. Inevitably, the soldiers will open fire, and you’ll have to take them all out (except Captain Diaz, who’ll run away). Once the area is clear, untie Felicia to complete the mission and unlock Sergio.