can someone explain why when I summon this w/ cyberload fusion when my opponent declares an attack it's unable to destroy spells/traps because it "missed it's activation timing"?
Did something else occur at that time? Like your opponent activates an effect during the attack, and you chained to it with Cyberload Fusion?
In that case, your opponent's effect will resolve first after Cyberload Fusion, since Rampage comes after the chain is already made. Rampage has to wait first, and by then it already lost the summon window to activate its effect, hence "miss the timing".
Remember that Rampage's spell/trap destroying effect is a "When....You Can", so its summon has to be the last thing to happen (nothing else in that moment is resolving too).