@Geo65,

802.11 AC was the generation immediately following the Wireless-N generation. 802.11AC actually accounted for devices not having the new standard and allowed for automatic backwards compatibility if a device did not support the AC protocol. I would imagine that your devices and router had to go through a negotiation process before you added n/ac mixed into the router settings.

I believe that this has become very common among the wireless standards released since, as it serves to exponentially make the setup process easier.

Out of curiosity, were you able to notice any lag, buffering, or delayed connections when it was operating on only AC?

I found this on an extremely old reddit post, but still may help clarify a few things: 

""

802.11AC is fully backwards compatible with 802.11A and 802.11N in the 5 GHz band.

802.11AC isn't even available for the 2.4 GHz band, which means that a router that only did 802.11AC wouldn't even be compatible with B and G devices.

Most 802.11AC routers are dual band, which means they do 802.11A/N/AC on 5 GHz and 802.11B/G/N on 2.4 GHz.

Hope this clarifies things a bit.

""