I recently bought a refurbished Dell Optiplex 7020 desktop. Due to various connection issues (dropped connection, slow speed, etc.), I bought three USB wifi adapters: a TP-Link TL-WN725N (N150 single band), a TP-Link Archer T2U Nano (AC600 dual band), and a refurbished Linksys WUSB6300-RM2 (AC1200 dual band). I also have a Synology DS918+ at home. I map the Synology to the desktop.
The issue I have is whenever I transfer large files (more than 1.5GB) between the desktop and the NAS using the dual-band adapters (either TP-Link or Linksys), the wifi connection would drop after some time (15-20 seconds perhaps). Once the wifi dropped, I lost internet as well. Re-plugging the adapters connected me back to the internet but not the mapped drives on the NAS. I can, however, access the NAS in a browser. Switching the adapters from USB 2 to USB 3 in the front panel seemed to fix the issue. Using the USB 3 on the back of the desktop had the same problem. For my peace of mind, I plugged it in the front panel. However, the issue came back after just one day. The wifi only dropped during LAN file transfer.
For comparisons, I used the lower speed TP-Link single band adapter for several days. Other than the speed, it never dropped connection during LAN file transfer. I also map the NAS to a Surface Pro 3. Surface Pro 3 is equipped with a dual band adapter and I never experience any dropped connection during LAN file transfer for several months now.
At first I thought the culprit was the USB wifi adapters but most likely I was wrong given that I have this issue with two different adapters and switching from USB 2 to USB 3 seemed to fix the problem, at least temporarily.
I can certainly use the lower speed single band since so far it works fine. But the speed is 6-7 MB/s versus 30 MB/s for dual band.