Problem with oldest wifi clients?

I have a Samsung np550pc-s05tr(made in 2013) laptop, my laptop wifi card is Intel® Centrino® Advanced-N 6235 Dual Band and I am experiencing this problem with it. It connects to the router with 300mbps, the distance between me and the router is 1.5 meters and there is no wall in between. Also, I have no problems with my tplink ax1800 device, which I use for 4a gigabit edition stock rom and other tests. This speed fluctuation problem only occurs when openwrt is installed.