Help, this was supposed to be easy !

I have just bought a set of 3 deco E4 and I have no idea how to connect them, I have a BT smart hub2 - do I connect them to the smart hub or do I connect them to the box in the wall ?

I've tried both and connecting to the router my speeds are shockingly low 4kpbs

If I connect to the wall it's asking for  pppoe ?? 

I wish I hadn't bothered, please can some one tell me what to do in simple English and step by step