I just bought an AC750 (couldn't pick it up from the above list) and set it up ok with the Tether App. It created two extra networks for my 2.4 and 5ghz both named ####_EXT. 

My question is whether I actually have to be on those in order to get the benefits of the extender, or if it will work even on my primary network not with the _EXT? If this is indeed the case that I need to be on it, how would I be able to switch this, as there are parts of the house where it's just better to be on the main wifi from the router and I don't want to have to switch when I walk to the back of the house with my phone. 

Thanks in advance,

Eric