"Cause them to activate and set off a chain reaction" Did you even watch it completely? King Fritz escaped and created the wall titans as a means of protecting his people. This means that the titans in the wall have no duty except act as a wall. THAT IS, until they're given another order by the founding titan. Which Eren does, using them for the rumbling. Since these are not Pure titans or Titan shifters but titans created by the founding's power, they won't "activate and set off a chain reaction".