Two defects, both specific to running this sample on a XIAO rather than a DK,
and both only on the border router side -- which is why the link was so lopsided
and why a Matter device could attach but not sustain SRP updates.
The coprocessor sample sets CONFIG_GPIO=n, which is reasonable on a DK whose RCP
has no GPIO-controlled RF hardware. This board selects its antenna path with
one: P2.03 powers the RF switch and P2.05 picks ceramic or external, declared as
regulator-fixed nodes with regulator-boot-on. With GPIO off, REGULATOR_FIXED
cannot build (it depends on GPIO), so those nodes got no driver, neither pin was
ever driven, and the border router ran with an unpowered antenna switch. The
devicetree looks correct throughout, which is what made this hard to see.
Separately, Zephyr defaults OPENTHREAD_DEFAULT_TX_POWER to 0 dBm and the sample
never overrides it, while Nordic's Matter samples use 8 -- so the border router
was transmitting 8 dB weaker than the devices talking to it. That asymmetry hits
beacon responses during a joiner's active scan, which is exactly what kept
failing.
Measured: -93 dBm at close range before, -71 dBm after, against a -101 dBm
sensitivity floor.
