My guess is that the Wireless antenna wire has a break in it. You can just replace it with a new antenna wire. Doing the antenna wire replacement, will mean taking the back cover off, the bezel around the display and then pulling the old antenna out, and fishing the new one up. It is easier, to just leave the old wire in place, tape the new wire to it, then use the old wire as a pull string.

I actually just placed my second antenna inside the case to the right side, and it works very well. I am getting 300mbps, no probblems like I was with the single antenna. Try just attaching the new replacement antenna first, then see what happens. Otherwise, you could just go with a USB adapter that is Wireless-AC 2.4ghz/5ghz capable, and forgive tearing apart your unit, to replace the antenna.

I got these for adding a second antenna, on two units, that we stuck the Intel 7260 Dual-Band Wireless-AC mini-card into. Due to the units only had one antenna for the Realtek wifi adapters. : Pair Laptop Wireless Mini PCI PCI-E Internal Antenna : Computers & Accessories

Now if you want the mini-card, if you do not currently have Wireless-AC, you can get it from for $28.