Pablo wrote

For instance, in 2.4ghz, I want to allow a channel selection of 1, 6 or 11 for obvious reasons, but I don't seem to be able to do this.  As a result I am seeing APs locking on mid frequency channel (9 and 13 seem to be very popular).

That's correct, if you select automatic channel selection, the controller chooses the best channel out of all available channels.

Note that so-called "mid-frequency channels" (all are "mid-frequency", even 1 and 13, since they all overlap in the 2.4 GHz band and there are still frequencies below channel 1 and above channel 13!) might even be the best if rogue APs allocate a distant-enough channel. There are three methods to choose the best channel known as the "rule of 4", "rule of 5" and "rule of 6":

In 802.11n mode with channel width 20 MHz @ 2.4 GHz the minimum distance to get non-overlapping channels is 4 channels. In 802.11b/g and in mixed modes it is 5 channels.

Thus, if some rogue AP is using channel 5, you could either select channel 1 or a channel greater than 8 to get a non-overlappig channel if both APs use only 802.11n mode.

In 802.11b/g/n mixed mode, minimum channel distance should be at least 5 (1-6-11) and in some countries it could even be 6 (1-7-13) if channel 13 is allowed to use. But it could also be 2-7-12 or 3-8-13 if you follow the rule of 5. With the rule of 6 you only have two choices for what you call "mid-frequencies": 2-8 or 7-13.

Why would you want to avoid channels outside the 1-6-11 scheme? Can't imagine a good reason for it.

See the following picture for a very bad channel allocation of WiFi devices all using only 1-6-11:

In this case I would fix a new AP at either channels 3, 4, 8 or 13, but not 1, 6 or 11. There is less interference at the edge areas of the nearby 20 MHz-wide channels. So, if an AC50/500 uses channels 9 or 13 if it senses less interference, there is nothing wrong with its selection IMHO.

What's more, if people use 40 MHz channel width, their APs allocate 8 channels. Not much non-overlapping channels left to choose from for another AP and if it's using 40 MHz channel width, too, you have no change except to use overlapping channels.

Pablo wrote

It IS accepted that in the 2.4ghz spectrum that ch 1, 6 and 11 ARE the channels to use.  These are NON-OVERLAPPING channels.

This is wrong. Channel 1 always overlaps with channel 2 and 3, and the two channels below the mid-frequency of 1 if using 20 MHz channel width (as do most devices). Channel 6 overlaps with 4, 5, 7 and 8. If you want to get true non-overlapping channels 1-13, you have to set 5 MHz channel width, which is possible most often with Outdoor APs only. But you can't change other people's APs.

If any AP outside your control uses channel 2 or 3, you will have a rogue (or "foreign" or "other people's") AP overlapping with the channel of your AP using channel 1! In fact, all 2.4 GHz channels overlap, and this is the root of the problem everyone faces, since you cannot force anyone to only use 1-6-11 (or 2-7-12). Maybe a picture helps to clarify that all channels do overlap:

Note that APs with a weak signal below -80dBm won't interfere too much with your AP even if they overlap your channel.

It's only that your APs in a defined area should be set to different, non-overlapping (from their point of view!) channels. If using channel 2, 7 and 12 for your APs, then they don't overlap relative to each other. But they still do overlap with other devices not under your control as do channels 1-6-11.

In short, there is no such thing as "non-overlapping channels" if you take into account all APs around you.

Because you mentioned urban areas, I have 280 APs around my office in the 2.4 GHz band which are detected by our AP and I still get a fair amount of AirTime for our outdoor AP. There is not even the slightest change to use a "non-overlapping channel" with 280 devices!

As for locking the AC50, file a feature request with TP-Link support if you need this functionality.

Yes, but best practice for me in the EU still is the "rule of 5", not some fixed channels others insist on b/c in other countries they don't have the change to use channels up to 13 and are limited to the 1-6-11 scheme. I trust my Netspot measurements if in doubt.

Regarding feature requests, either use the Requests & Suggestions forum or file a ticket with local TP-Link support.