That is one the better wifi nic cards...the external antenna help a lot. Now it will still be no better than a wifi5 model with a 1200 number if your router can not actually use the 160mhz wide radio channels.
I would check what radio band your 2 devices are using. Maybe one is connecting on 2.4 and the other on 5. This is one of the key reason you make the SSID names different so you can force the connection where you want it.
Both 2.4 and 5 should have massively more bandwidth. Note be very sure you get the terms correct. Steam many times displays the rates in MEGA BYTES/sec where when you write it Mbps it means Mega bits/sec.
I would try switch between 2.4 and 5 and see if it is any different. Check that all the antenna connections are tight, with that fancy card you have a few more to check. Try to move the antenna block around a bit. On top of the case tends to be the best spot for most people.
After this you start wondering if it is drivers or something. I assume it happens on speedtest and when you say download a large file from microsoft?. Have you tested with a ethernet cable to see if it is only wifi that has issues.