i am stumped

hi all I have a xfinity gateway all in one wifi built in it.
this what was going on in the back room of the house my wifes laptop would lose wifi all the time so I got a new gateway thinking the first one was bad but it dose the same thing . so I got a asus 802.11ac rt-56u router thinking the laptop was to far away from the gateway so I got it all hooked up and it wall stay connected but the speed is slow. the gateway well get about 40.00 dl and 11.00up asus well get 18.00 dl 12.00up and just to make things interesting my Samsung gs4 well get 56.00 dl and 12.00 up so I got into the settings of the router and tried all the channels 1 to 11 . 20megaherts to 40 2.4 g to 5 g
I can get the laptop fast but the gs4 well be slow like 3.0 dl 6.0 up. sorry for the bad grammar ijust don't what I can do now
ps I did all updates on the asus router.

OK 3 dumb questions should Idisable the gatway wifi?
can i put the wired computers back to the gateway so i can move the asus router about 20 feet away I only have one 20 foot cable?
could the house braker box afect the gatway and router? They are setting right inside on the same wall.

Sounds like something might not be right in the settings. Im not sure how asus routers are set-up on their settings page; so you will have to go into the web based interface(the router must be connected to at least 1 ethernet connected computer to do this) and once in there; find the page that should allow you to change the channel on which the router broadcasts and change the channel to like anywhere from channels 5-8(preferably in the middle as not many people know to use a different channel). Try this and post back if it doesnt help. And yes; the breaker box could be affecting it; as anything with electrical power sources(which covers a lot i know) can cause interference. So yes i would try and get the router as far as feasibly possible away from the breaker box.