docs(openwrt): document STA-on-same-radio channel capture; warn in fips-mesh-setup

Field-found third silent-non-peering cause: a client (sta) interface
must follow its upstream AP's channel, the whole radio follows the
STA, and a mesh pinned to a different channel never joins — visible
as 'type mesh point' with no channel line and an empty station dump.
It also does not recover when the STA disconnects (wifi reload + fips
restart needed), so a roaming uplink is fundamentally incompatible
with a fixed-channel mesh on the same radio. Add to the guide's
triage and constraints, and make fips-mesh-setup warn when the target
radio carries a STA.

docs/how-to/set-up-80211s-mesh-backhaul.md

@@ -174,7 +174,16 @@ in this order:
 1. **Channel mismatch** (the most common cause): compare
    `iw dev fips-mesh0 info` on both routers — mesh ID *and* channel
    must match exactly.
-2. **Is the other router transmitting at all?**
+2. **The mesh interface never joined** — `iw dev fips-meshX info`
+   shows `type mesh point` but **no channel line**, and `station dump`
+   is empty. Usual cause: a client (`sta`) interface on the same
+   radio. A STA must follow its upstream AP's channel, the whole
+   radio follows the STA, and a mesh pinned to a different channel
+   silently stays down. Check for a STA sharing the radio
+   (`iw dev`, look for `type managed` on the same phy), compare
+   `iw dev <sta-iface> info | grep channel`, and re-pin the mesh
+   channel to match — on every backhaul router.
+3. **Is the other router transmitting at all?**
 
    ```sh
    iw dev fips-mesh0 scan | grep -i -B4 "MESH ID"
@@ -186,7 +195,7 @@ in this order:
    CAC wait, and confirm the country code is set
    (`uci get wireless.radio1.country`) — an unset regdomain can
    block channels entirely.
-3. `logread | grep -iE "mesh|fips-mesh0"` on both sides.
+4. `logread | grep -iE "mesh|fips-mesh0"` on both sides.
 
 Then the FIPS layer on top:
 
@@ -217,3 +226,12 @@ sight.
 - **Radio links are lossy.** A neighbor at the edge of range will
   form an 802.11s peering yet deliver a fraction of its frames.
   Expect link-quality effects that don't exist on wired Ethernet.
+- **A client (STA) uplink on the same radio owns the channel.** The
+  STA must follow whatever channel its upstream AP uses; every other
+  interface on that radio follows the STA. A mesh pinned to a
+  different channel silently never joins, and it does **not** recover
+  when the STA disconnects — a `wifi reload` (plus a fips restart) is
+  needed. A *roaming* uplink (travel-router / hotspot-chasing setups)
+  is fundamentally incompatible with a fixed-channel mesh on the same
+  radio: dedicate the mesh to the radio the STA never uses, and treat
+  any mesh sharing a STA radio as best-effort.

packaging/openwrt-ipk/files/usr/bin/fips-mesh-setup

@@ -152,6 +152,21 @@ if [ -z "$CHANNEL" ] || [ "$CHANNEL" = "auto" ]; then
 	echo "  uci set wireless.$RADIO.channel='36' && uci commit wireless && wifi reload" >&2
 fi
 
+# A client (sta) interface on the same radio follows its upstream AP's
+# channel and drags every other interface with it — a mesh pinned to a
+# different channel silently never joins, and does not recover when the
+# STA disconnects.
+for s in $(uci show wireless 2>/dev/null | sed -n "s/^wireless\.\([^.]*\)\.mode='sta'$/\1/p"); do
+	if [ "$(uci -q get "wireless.$s.device")" = "$RADIO" ]; then
+		echo "Warning: $RADIO also carries client interface '$s' (mode 'sta')." >&2
+		echo "The whole radio follows that STA's upstream channel — a mesh" >&2
+		echo "pinned to a different channel stays down silently. Align the" >&2
+		echo "mesh channel with the upstream AP, or put the mesh on a radio" >&2
+		echo "without a STA (a roaming uplink is incompatible with a" >&2
+		echo "fixed-channel mesh on the same radio)." >&2
+	fi
+done
+
 # ---------------------------------------------------------------------------
 # Network: unmanaged interface so netifd brings the netdev up. No IP config —
 # the FIPS Ethernet transport speaks raw frames on it.