DIAGNOSIS The primary instability is in the plastic neck-to-stand connection or the desk surface itself. The stand is lightweight by design.
FIX Remove the monitor from the stand. Tighten the single screw that connects the metal neck to the plastic stand base. Reattach the monitor. Place the monitor on a completely flat, hard surface. Avoid desks with a slight flex or padded mats.
IF THAT DIDN'T WORK The wobble is inherent. Consider using the 100x100mm VESA mount. Buy a third-party monitor arm or sturdy stand. This completely replaces the included stand and eliminates wobble.
HEADS UP Do not over-tighten the screws connecting the monitor to the neck. You can crack the plastic housing. The connection is meant to be hand-tight plus a quarter-turn with a screwdriver.