Adapter constantly switches to N

I have to assume you are using two different SSIDs for the two different standards - delete the network entry for the "n" SSID and the computer will be unable to connect to it. You may find at that point that what happens is you periodically loose your connection to the "ac" network - which would then reveal the true cause of the "switching" - a poor quality signal on the "ac" network. I think it's time for the industry to recognize that each successive step brings additional complexity and that the users in general (not specifically you) will need to know more about the technology in order to make it function - it may also be time for the users to recognize that they have to learn more about the technology or call a professional. 802.11ac is a 5GHz only standard and as such subject to the increased attenuation of the higher frequency band - it simply does not penetrate walls and furniture to the same extent as a 2.4 GHz signal would - 802.11n is a dual band standard, and supports both 2.4 & 5 GHz.