I have a deco X20 pack (3) and I would like to know which system or app maybe android I can use to check the best position where to put the other two decos Thank Hispa

I but today a Deco X20 system (3pcs). So i ask some suggestions to set properly: 1 - i have in my house a modem/router with many devices connected in lan... so is better tha deco is set as Router or A