Well, that’s the "question of the year. 
I can’t tell you how much time we have spent trying to optimize wi-fi over the years – and at dozens of locations. @MarceloBarros and @ChristopherSpitler both bring significant experience and I can say what they say (above) has great merit. The references they cited are also good. I can also say when troubleshooting a goofy wi-fi situation a year or two ago I submitted a ticket to Peplink where in a minor AP FW issue was subsequently identified (now fixed) and I took the opportunity to ask for Peplink’s thinking about channel use. Essentially, I was advised to use the same channels on APs within hearing distance of each other unless there was an issue in doing so.
A gratuitous aside, which you may or may not find to be helpful: As I write this, I find myself in an extremely dense wi-fi environment. The three Peplink APs (using the excellent, older wifi5 versions at this location) see dozens of foreign APs with signal strengths of -80dBm or better. (These are a mix of various owner-furnished routers/APs and AT&T supplied Humax BGW320s, with the occasional goofy wi-fi extender tossed in for spice.) In brief, it is a mess, RF-wise.
I’ve played with this a bit as time permits. The “quasi-Pareto-optimal” solution at this location is presently to:
- Let all three APs use the same frequencies on both 2.4 and 5GHz. Do not set it to “auto” as the Humax devices are reasonably smart and that is their default (few people will change this.) I want as many of them as possible to see our system – always – and avoid us to the extent possible. I don’t want to set a setting of Auto here to let us become a “moving target.”
- Use 20MHz on 2.4GHz. 2.4GHz is primarily used for IoT/M2M devices. Reliability is important and blinding speed is not. If “smarter” devices need 2.4GHz they’ll still work, just not as fast as if on 5GHz. And, they won’t be there long (see below.) IoT/M2M clients are often incredibly dumb and very often have never had a FW upgrade – and never will. The dumber they are the more they will like 20MHz channels. And, their reliability is a key consideration.
- Use 80MHz on 5 GHz. I’ve tested both 40 and 80 and don’t see much difference in the present RF-rich environment, although I am very well aware of the arguments for 40MHz channel width – which @ChristopherSpitler sets forth succinctly.
- The “Preferred frequency” is set to 5GHz. (This helps “higher-end” clients communicate faster; see point 2, above.)
- We’ve tested (here and at a few other locations) the “Assisted Roaming” feature in Peplink’s recent FW. In the present environment, “aggressive” seems to work best. “Moderate” works OK “Off” is not so good. (We, and others, raised issues with Peplink regarding clients “hanging on” to APs for far too long in a degraded RF environment. This relatively new parameter helps – a lot.
- Minimum signal strength and system behavior regarding how the AP will respond with lower than desired signals are very important. Here are the present settings here:
We’ve found, through experimentation, that these work at this location. One thing to note: If a client on 5GHz gets really weak we want to disconnect it rather than drag the bit rate on that band down – which is why the “Disconnect” box is checked.
I’m sure others have their own thoughts on this but I thought I might share a solution that has worked in the present “RF rich” environment. Every location is different – some dramatically so. (Our solution for a 200,000sq ft warehouse is nothing like what I have set out above.) 