Remnant 2's Forgotten Kingdom DLC includes a co-op puzzle similar to the one in the base game, with the Pan War Band as a reward. The items from these puzzles are only really useful if you're playing in a group, so you won't be missing much by skipping them.
Remnant 2 How to Get PAN WAR BAND (Infested Abyss Co-Op Puzzle) | The Forgotten Kingdom DLC
You'll find the Pan War Band in the Infested Abyss in Remnant 2's Forgotten Kingdom DLC. From the main entrance to the area, head straight ahead, sticking to the left wall until you find a ledge with a small path below it. Drop down into the area and follow the underpass, carefully descending until you reach a large hallway surrounded by pillars.
The Pan War Band is located in the upper part of the underpass, which requires two players to access. Here's what you need to do:
The Pan War Band increases Movement Speed, Reload Speed, and Fire Rate depending on how many players in the party are wearing it at the same time. This is how much the players' stats are increased: