Yantras will really work,but it has to be Given the life with Mantras properly.Giving life to a mantra & the Puja done has to be correct.If you learn to make Yantras then it may work.I am not sure if Yantras bought from Marketplace work,because why cant the person who makes & sells these Yamtras make Lakshmi or Kuber Mantra do it for himself & automatically attract wealth.
As you may be aware Sri Adi Sankaracharya is supposed to have Installed the Dhana Akarshan Yantra at Tirumala & u can see the effect of it.
So if a Yantra Vendor can do it say Lakshmi Yantra he will not sell it .So learn Yantras from a True Guru,that will work