Any recommendations for a small office WiFi router?

Preferences:
* 2.4GHz (for older devices) and 5GHz (for speed and no interference).
* Single router (I hate managing multiple boxes or diagnosing problems due to one box going glitchy).
* Not too hard to setup (each hour setup is costly to us - my time is valuable to business).
* Stock firmware (I have used variety of open source firmware for home and friends, but it just costs too much time).
* Prefer something popular (oddballs are hard to maintain and hard to justify to colleagues).
* I would expect to pay many hundreds of $, although we can afford low thousands of $ if convinced it will be worth it.

Office:
* Easily covered by a single device.
* Currently 6 laptops on WiFi, 15 devices (and growing so need headroom).
* Also have wired network, but people prefer convenience of WiFi.
* Currently using NetGear N600 router but it glitches every 2 to 8 weeks and it gets blamed for other problems (because glitches are intermittent and not same every time).
* Tried Netgear WRT1900AC - but it dropped WiFi network every few hours.

Reliability is #1 - just has to truck on every month without problems (network downtime costs us, but more importantly if not 100% rock solid then WiFi gets blame for other problems which leads to other ramifications).

PS: I would also consider recommendations for an *individual* contractor in Christchurch or PM me.