Fiber wifi problem

Hi I just want to make sure before I do anything, because everyone's situations are different. 

So I had read few forums about having different router, because ATT BGW210 would NOT cover a thing for my devices. 

I recently moved and my previous owner of the house had set the fiber at basement for some reason, so I cant use ethernet either.

and also I bought the nighthawk coverage expender, but its not doing much of the work because the modem of the WIFI is bad. 

and here is the questions.

1. So I read few thing on google that say I could buy the router and connect with the "pass though mode". does this will affect on speed of the Wi-Fi? will it get faster? is there any possibility that ATT device does IP block? if its not possible, can i get any upgrade on router? the Gateway?
2. Internet itself is just fine. they cover around 980~950 on income and outgoing internet. but after going thought the "home network hardware" it slows down to 200 (right next to the device). is it the modem's problem right? 

Thanks guys, please help :( its been real pain to do online class and games.