AC1900 as access point now allowing only 802.11a connections

We purchased this Extender/Access Point on March 2 of this year. Until about two and a half weeks ago we were no longer able to download real-time 1080p video from it because the transfer rate was too slow, far too slow. After a great deal of testing and experimentation it has become clear that the problem is that our laptop, Windows 10 with an ASUS AC56R high-speed USB adapter, was seeing the 5 GHz band as 802.11a protocol, not 802.11ac protocol. The laptop and adapter work fine when set to the ac-protocol band on our router so the problem seems to lie with the AC1900 in access control mode. We have made no changes to the configuration of the AC1900 whatsoever since we initially set it up in March although we did update the firmware once quite some time ago.

I hope that someone can help us figure out what is going wrong and how to fix it!

Thanks in advance!