Built a new pc, had to lower my "Speed and Duplex" to get my pc to recognize my ethernet coming from my modem/router

I built a new computer recently and when I initially plugged in my ethernet cord into my pc (CAT7) it would not recognize it and it just said "not connected." I googled and researched and it turns out I had to lower my "Speed and Duplex" in my ethernet port's settings to have my pc recognize the cord was even connected. It worked but my speeds are drastically dwarfed due to this. I usually would get anywhere from 35-50mpbs (AT&T fixed wireless) and now I'm getting about 6-7 mpbs and if I turn up the speed and duplex setting up any more it will not even recognize that anything is plugged in. My modem and router are about three years old. Has anyone else ever had this problem and should I just contact AT&T for a new modem/router? A few posts on reddit signaled that new/replacement equipment have fixed their issues. I just want the speed on my pc that I'm paying for and I want to be wired in. Thank you!