This does not strike me as specific only to this router, but has to do with a configuration setting around 20MHz vs. 40MHz channel bandwidth.
I picked up an Asus RT-AC68R a while back and it’s been fantastic. One bit of behavior I wasn’t crazy about was the periodic – and somewhat frequent – dropping of the wireless connection. This happened with multiple devices (all of which happened to be Apple). I read around a bit online and there were a few suggestions, ranging from configuration to third-party firmware, but the one I homed in on had to do with channel bandwidth. I live in an urban environment and there are many APs within range of me. Evidently, by default, the Asus comes configured with 2.4GHz to use a channel bandwidth of 40MHz (or 20/40) as opposed to a strict 20MHz bandwidth, and this has been identified as a potential issue in crowded AP environments. I limited the channel bandwidth to the more concise 20MHz and my dropped connections have disappeared in the weeks since.