Cant change wifi speed on Range Extender N300 from 145mbps to 300mbps?

The main wifi-router and the wifi-modem are both on the living room, next too each other and they are 10-15 meters from the wifi-extender. My room which contains a PS4, laptop and an iPhone 6, are 3-4 meters from the wifi extender.

I've had my Netgear Wifi Router Extender N300 for a year now and it has never been this slow. It started two weeks ago when I was playing GTA Online on my PS4 and I had the worst connection ever. The whole game was lagging, on online and all other online games were also lagging on my PS4. I had this problem for 3 days so i decided to restart my extender. But that didn't fix the issue. All my online games on the PS4 were still lagging like  and the internet was also slow on my iPhone 6 and my laptop. The coonection was ****!

So i decided to reset the whole extender to factory settings and after I went through the installation process the internet was up and running but still a little bit slower than normally. I opened the wifi extender settings and saw that the wifi speed was set to 145mbps but it should be set on 300mbps...

BUT I CANT CHANGE IT TO 300MBPS CAUSE THE  BUTTON IS STUCK/UNCHANGEABLE??? IT WONT LET ME CHANGE IT TO 300MBPS???