Deco M4 V2.8, All child node are connecting to Main router.

I having 3 DECOM4 V2.8 setup.  Main router connted to internet provider's router and other 2 are setup as child units using DECO app.

But both child node are always connecting to main router and node located the logest distance giving vey less speed.

3rd router should connect to 2nd router (placed between main router & 3rd router). Any suggestions.