AC1900 C7000 router

I just switched to Xfinity service to get faster internet speeds. When I directly plug into ethernet port, my PC shows a download speed of 107.8 down and 5.7 up. Using wifi I get 38.4 down and 4.57 up on my PC; 118.68 down and 6.19 up on my cell. I would like to use my C7000 AC1900 Nighthawk for wireless, but when I do, my download speed drops to 2.97 and 2.81 up.  It doesn't seem to make a big difference whether connected via wifi on my pc or cell phone. Even when I directly couple the Nighthawk to the PC still less than 3mp down. Looking at the attached devices, I see the same download and upload speeds as seen by my PC. I've just downloaded and installed the latest update and tried all of the suggestions on this subject from the community. Is there something that I can look at?  Thanks for your help.