FAP-231K (Japan region) limited to W52 channels only on FortiOS 7.4.x - resolved on 7.6.3, any plan to backport?

Sharing this in case others run into the same thing, and to ask whether
there is any plan to address it on the 7.4 branch.

## Summary

On a FortiAP-231K with region code "J" (Japan) managed by a FortiGate
running FortiOS 7.4.x, only W52 channels (36/40/44/48) are selectable
in the FortiAP Profile "Set Channels" screen.

All DFS channels (W53: 52-64, W56: 100-144) and UNII-3 are greyed out.
The "Toggle DFS Channels" and "Toggle Weather Radar Channels" buttons
are disabled as well.

In Japan, 5 GHz regulations allow 20 channels in total (W52: 4, W53: 4,
W56: 12). Being limited to 4 channels makes high-density design very
difficult, since the channel reuse distance collapses.

## Environment and test results

- FortiAP model: FAP-231K
- Region code: J (Japan)
- Country/Region on FortiGate: Japan
- Management: FortiGate-managed (CAPWAP)
- Radio: 5 GHz, 20 MHz width

I tested every FortiAP firmware from 7.4.5 through 7.6.5.

Result: as long as the FortiGate is running FortiOS 7.4.x, the behaviour
does not change at all. W52 only, regardless of which AP firmware is
loaded.

Upgrading the FortiGate to FortiOS 7.6.3 makes the issue disappear -
W53 and W56 become selectable as expected.

So the AP firmware is not the deciding factor here. The FortiOS version
on the FortiGate side is.

## What the release notes say

DFS channel support for this specific model and region was added on the
FortiAP side in FortiAP 7.6.3, listed under "Region/country code update
and DFS certification":

  Bug ID 1188974 - Enable DFS channels for FAP-231K with region code "J"

So the AP side has supported it for a while. The channel list in the GUI,
however, is rendered by the FortiGate, and that is where the limitation
sits.

## What TAC confirmed

I opened a ticket and got the following:

1. FortiOS 7.4.x does not appear to include the updated channel
   information for this model / Japan region combination, which is why
   only 36/40/44/48 are selectable.
2. There is no public documentation indicating this change was
   backported to any 7.4.x release.
3. No minimum FortiOS version is documented in the release notes for
   Bug ID 1188974.
4. Upgrading to the 7.6 branch was recommended if DFS channels are
   needed.

## The part that does not add up

TAC also stated that for FortiOS 7.4.5-7.4.12, the recommended FortiAP
firmware for FAP-231K is 7.4.7.

I tested that pairing as well, and the behaviour is identical - W52 only.

In other words, the *recommended* firmware pairing still cannot use any
DFS channel in Japan. This is not caused by running a mismatched
firmware combination. FortiOS 7.4.x simply does not carry the channel
table entry for this model and region.

## Why this is worth flagging

Older FortiAP release notes used to state the FortiOS requirement
explicitly for this kind of DFS enablement. For example, the entry
enabling DFS channels for FAP-231F region code "A" included a note that
the FortiGate needs to run FortiOS 6.4.6 / 7.0.0 or later.

No equivalent note exists for Bug ID 1188974. There is also nothing in
the Compatibility Matrix indicating that this model will be limited to
4 channels in Japan when managed by a 7.4.x FortiGate.

There was no way to find this out from public documentation. I only
discovered it after purchasing the hardware and testing it in-house.

## Questions

1. Is there any plan to backport the channel table update to the 7.4
   branch?

   I would rather not move production to 7.6.x yet. The branch still
   feels relatively young, and for an environment like this I would
   prefer to stay on a more mature release train. Staying on 7.4 while
   having DFS available would be the ideal outcome.

2. If a backport is not planned, is there any official statement I can
   reference? Right now the only source I have is a support ticket, and
   I would like something citable when explaining the constraint.