Changing to teh Billion - it might be better, or it might be worse.
As for teh 2700, it depends on which 2700 you have - some can be tweaked to increase wireless power, but not all can.
You best bet is to get another WAP in some form site it where you need coverage and connect it to the original router using a cat5e cable. Get another 2700HGV or 2701HGV of ebay and use that.
I have a 2701 feeding three additional 2700s which act as WAPs
M H C
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